In the next few lab periods, we will examine three eumetazoan phyla that are classified as deuterostomes. The term deuterostome means "second mouth" and refers to the fact that during development of the digestive system, the blastopore develops into the anus and the mouth is created secondarily. There are also developmental characteristics such as orientation of cells during cleavage and formation of mesoderm that indicate close relatedness of deuterostomes (Hickman et al. fig. 8-9). Members of phyla Echinodermata, Hemichordata, and Chordata are more closely related to one another than they are to any other organisms we have seen this semester.

Phylum Echinodermata [hedge-hog skin]. sea stars (starfish), brittle stars, sea urchins, sea lilies, sea cucumbers

Echinoderms are a very unique group of organisms usually found in marine environments. There are about 6,000 species of echinoderms, some of which are sessile. Although most are free moving, echinoderms have radially symmetrical bodies as adults and no head. This goes against the basic rule that mobile organisms are bilaterally symmetrical and exhibit cephalization. Echinoderms do have bilaterally symetrical bodies during the larval stage (bipinnarian larva) of development but then metamorphose into radially symmetrical adults. In fact, most of the evidence that echinoderms are closely related to chordates comes from characteristics observed early in development. Other characteristics of echinoderms are the presence of an endoskeleton and a unique water vascular system. The endoskeleton is composed of calcareous plates which often have spines that protrude through the epidermis. The water vascular system operates through water pressure to manipulate sucker-tipped tube feet used to hold onto the substrate and in locomotion. Although echinoderms do not have a circulatory system, they have a hemal system which is thought to help distribute food to various parts of the body. Distribution of other substances (nitrogenous waste, dissolved gasses) may be acheived through currents in the coelomic fluid.

Most echinoderms are dioecious. In general, echinoderms have tremendous powers of regeneration; sea stars and brittle stars can replace lost arms and sea cucumber can discharge internal organs as a defense mechanism against predators and then regenerate new internal organs.

Seastars have powerful commerical impact on shellfish industry being active predators on oysters, clams, and scallops.

Classes of Echinoderms:

Class Crinoidea - sea lilies; 5 arms with basal stalk; prominent fossil forms

Class Asteroidea - sea stars - broad base where arms attach

Class Ophiruoidea - brittle stars - narrow base where arms attach

Class Echinoidea - sea urchins and sand dollars; no arms, many sharp spines

Class Holothuroidea - sea cucumbers; no arms or spines

Phylum Hemichordata - [half chordates] acorn worms

The body plan of acorn worms has three body regions: a proboscis (burrowing structure), collar with mouth, and a trunk or body with gill slits and digestive tract. Members of phylum Hemichordata share many traits with Chordata, including a dorsal hollow nerve cord and pharyngeal gill slits. Acorn worms are found in marine habitats and are dioecious.

Formerly thought to be a member of Phylum Chordate, acorn worms were removed and placed in their own phylum due to absence of a notochord (see Phylum Chordata). The structure that was formerly thought to be a notochord proved to be an anterior extension of the digestive tract = a stomachord.

While a bizarre form, it is difficult to equate the adult acorn worm to the main line of chordate ancestry as was the case with the echinoderms - the tornarian larva of acorn worms however shows very distinct similarities with the bipinnarian larva of the echinoderms and represents the line of relationship. All other deuterostome characteristics also operate making the hemichordates position in the ancestral line of chordates as unquestionable.

Phylum Chordata [string] - sea squirts, sea lancets, vertebrates.

There are three characteristics that unite all chordates:

1. dorsal hollow nerve cord

2. pharyngeal gill slits sometime in life - allows water to be taken into mouth and then flushed out of the gill slits in the pharynx

3. notochord sometime in life - the notochord is a flexible rod use to support the main axis of the body.

Ancestry of chordates from earlier deuterostomes and origins of earliest chordates cannot be explained on the basis of adult features but rather on the basis of larval events. The phenomenon of progenesis, the appearance of adult features (sexual maturity) in the larval body form, appeared in the larval forms of echinoderms, hemichordates, and urochordates providing the main line relationship among non-chordate deuterostome - chordate groups.

Classification of Chordates:

Subphyla:

Subphylum Urochordata - Highly specialized, bizarre group off the main line of Chordate formation. All are marine and have a mobile larval stage and sessile adults. Young urochordates go through a larval stage that shows the 3 shared Chordate characteristics; the adults have no notochord or hollow dorsal nerve cord.

Subphylum Cephalochordata - small 2-4 inch marine organisms. Considered responsible for moving into freshwater environments and applying preadaptations for success. Successful inhabitation of freshwater and resulting anatomical changes produced the first vertebrate animals. Many of the characteristics of vertebrates are associated with the movement of cephalochordates to freshwater environments:

1. The developement of a pumping (glomerular) kidney for removing excess water that entered the body through osmosis.
2. The development of an exoskeleton of dermal armor to prevent excessive uptake of environmental water and to maintain internal salt reserves.
3. The developement of increased musculature to maintain position in fast moving water bodies and the development of improved internal skeleton to enhance muscular attachment.

Final product to these anatomical changes was a heavily armored, bottom dwelling, filter-feeding organism having no jaws, no paired fins, and a strong cartilaginous supporting rod running the length of the body = the first vertebrate.

Subphylum Vertebrata - vertebral column replaces notochord during embryogenesis

ultimately represented by 8 Classes:

Agnatha - [without jaws] first vertebrates, off-shoots of cephalochordate modification in freshwater. (lampreys and hagfish)

Placodermi - [plate skin] earliest typical fish forms, developed jaws, paired fins, segmented the body armour, increased body size, improved kidney system, skeletal system, digestive system and life cycle. (only vertebrate class considered to be extinct.)

Chondrichthyes - [cartilage fishes] specialized groups of fishes originating in freshwater and moving back into the marine environment. (sharks and rays). Only 1 or 2 species presently occurring in freshwater. Developed a unique method of maintaining osmoregulation in the marine environment.

Osteichthyes - [bony fishes] became the dominant group of fishes and animals worldwide in the Devonian = The Age of Fishes. Radiated worldwide in marine as well as freshwater. Lobe-finned forms provided preadaptations for origin of first terrestrial vertebrates = the amphibians.

Fish dissection

Amphibia - [dual life] first terrestrial vertebrates. Tied to water as a result of external fertilization of eggs. Not an eminently successful group because of its water restriction for reproduction. Not long after its origin, certain members developed a unique egg (Cleidoic) which allowed for internal fertilization and development on land. Thence arose first truly terrestrial vertebrates.

Reptilia - [to creep] first completely terrestrial vertebrate group. Possess the ability to spend the complete life cycle on land. Became the dominant organism group worldwide from 65-135 million years ago = The Age of Reptiles. Greatest adaptive radiation success ever witnessed. Changing atmospheric conditions, ice ages, competition from early mammal types all contributed to the downfall of the group. Unique features include ectodermal scales, molting of epidermis, cleiodic egg, four chambered heart, temperature warming behavior.

Aves - [birds] one of three successful groups coming out of the reptilian radiation (modern reptiles, birds, mammals). Unique features include feathers, hollow bones, flow-through respiratory system, no teeth, homiothermy, egg laying.

Mammals - [mamma = glands]. Another of the successful groups emerging from the reptile group. Unique features include dry, dead, scaly skin, hair, homiothermy, birth to living young, suckle young.

Fetal Pig Dissections:
respiration and digestion
circulation and urogenital

Progressive development of systems among Classes:
a. Integument -living epidermis underlain with dermal scales in fishes, to living epidermis in amphibians, to dead ectodermal and dermal scales in reptiles to feathers and hair with dry, dead, water proof skin in birds and animals.
b. Circulatory system -two chambered heart in fish to three chambered heart in amphibians to four chambered heart in some reptiles and four chambered heart in all birds and mammals.
c. Temperature control -ectothermy in fishes and amphibians to behavior controlled body temperature in reptiles to endothermy in birds and mammals.
d. Skeletal system -internal and external bone and cartilage skeleton in fishes to internal and external bone skeleton in reptiles to mostly internal skeleton in birds and mammals. Trend through the animal groups is toward fusion of skeletal parts.
e. Reproductive system -from external fertilization and development of fishes and amphibians to internal fertilization and eternal development of reptiles and birds to internal fertilization and internal development of mammals.

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