INVERTEBRATE EUCOELOMATES 



263 



it while it is held by the tentacles — they have no di- 

 gestive tract. Of course, very little is really known 

 about these animals, because they must be brought 

 up from the depths before they can be studied. Be- 

 cause embryos are found in the tubes that the adults 

 seem to form, it is believed that there is no larval 

 stage and that the tube acts as a brood site for the 

 developing individual. 



CHORDATE ENTEROCOELA: CHORDATA 



Perhaps the last animal phylum to evolve was the 

 Chordata. In spite of our personal interest, because 

 we are chordates, these animals are not necessarily 

 the most successful ones upon earth. You may recall 

 that we cannot make positive statements, because we 

 are unable really to measure success. If we use such 

 criteria as numbers of individuals or species and 

 amount of the world occupied, the arthropods are the 

 most successful group. 



In the remainder of this chapter we shall examine 

 the general nature of chordates and the kinds of 

 chordates without backbones, a group often called 

 the "Lower Chordates." 



CHORDATA (Chordates) 



Diagnosis: symmetry bilateral; segmented, but 

 may not be visible externally; enterocoelous; includes 

 the tunicates, lancets, and vertebrates; tunicates 

 may be blubbery, club-shaped, or globular; all pos- 

 sess a sac-like covering or tunic; include sea squirts, 

 which may be recognized by their habit of expelling 

 water when touched, and which range from trans- 

 parent to opaque and brilliantly colored; lancets' 

 bodies are fish-like, but possess neither scales nor 

 other complexities of vertebrates; marine, fresh- 

 water, and terrestrial; none are strictly parasitic. 



Perhaps the subphyla of chordates bear less re- 

 semblance to one another as adults than do the mem- 

 bers of any other phylum. It was already stated that 

 they are considered more closely related to the echino- 

 derms and hemichordates than to any other phylum. 

 But, why are they a single phylum? The chordates 

 are distinct in possessing gill slits; a notochord, a long, 

 flexible, rod-like structure from head to tail along 

 the back; and a dorsal hollow nerve cord along the back 

 and above the notochord. 



The chordate ancestor can be reconstructed from 

 our knowledge of the invertebrate chordates. Per- 

 haps it was most like the living tunicates, in 

 contrast to other living forms, but it was much less 

 specialized. The evidence suggests that the ancestor 

 was sedentary and possessed an elaborate gill struc- 

 ture for food gathering, an alimentary canal, and 

 reproductive organs, all enclosed in a soft body. For 

 distribution of offspring, there may have developed, 

 as in tunicate larvae, a propulsive tail with noto- 

 chord and swimming muscles. From this hypotheti- 

 cal larva, fossils and developmental patterns show a 

 small step to the living chordates. 



Subphylum TUNICATA ( = UROCHORDATA) 

 (Tunicates) 



Diagnosis: larvae minute, free-swimming and tad- 

 pole-like, with notochord and nerve cord; gills gen- 

 erally involved in filter feeding; adults sessile; adult 

 body irregular, tubular to somewhat spherical, gen- 

 erally blubbery to club-like, covered by a transparent 

 to opaque tunic, without notochord or nerve cord; 

 strictly marine (Figure 15.10). 



CLASS LARVACEAE (Larvaceans or Tadpole Tunicates) 



Diagnosis: minute to | inch, transparent, tadpole- 

 like tunicates of persistent larval form, free-swimming 

 marine plankton that have a jelly-like tunic they may 

 leave to secrete a new one; microscopic food is filtered 

 by tunic structures, the necessary water current for 

 filtering being produced by moving the tail; usually 

 hermaphroditic, but with cross-fertilization, in- 

 dividuals first functioning as males and then as 

 females; open ocean forms. 



CLASS THALIACEA (Chain or Pelagic Tunicates) 



Diagnosis: small to 4 inches, colonies much 

 longer; body cask-like or spindle-like with anterior 

 and posterior openings, ringed by muscles that may 

 provide contractions necessary for jet-propelled 

 swimming, others transported only by currents; soli- 

 tary or colonial, colonial forms generally arranged 

 as chains of individuals; tunic permanent, adults 

 without a tail; filter feeders on microscopic life; 

 hermaphroditic, usually with difTerent generations, 

 hermaphroditic adult by sexual means produces a 



