260 



CHAETOGNATHA THROUGH LOWER CHORDATA: 



univalve mollusks. These starfish start Ufe as males, 

 but later become females. 



Starfish present little meat that could be used for 

 food, hence even primitive people seldom eat them. 

 Perhaps their closest relationship with modern man 

 lies in the damage they cause in oyster beds. Oyster- 

 bed owners not familiar with the regenerative powers 

 of starfish have had real problems with these oyster 

 predators. Bed owners have gathered the starfish in 

 the beds, chopped the starfish in two, and cast the 

 remains back into the oyster beds. Rather than 

 destroying the starfish, this practice results in twice 

 as many, because each part regenerates the missing 

 portions and becomes a complete starfish. Now, 

 most oystermen either collect and boil the starfish, or 

 kill them by sprinkling lime on the beds during low 

 tide. 



CLASS OPHIUROIDEA (Brittle and Basket Stars) 



Diagnosis: body star-shaped, but arms sharply 

 marked off from a central disc; generally five arms; 

 without longitudinal grooves on underside of arms, 

 or with grooves covered by small plates; arms un- 

 branched (brittle stars) or branched (basket stars) 

 (Figure 15.5). 



The brittle and basket stars are the most mobile 



Figure 15.5 Ophiuroid type: Ophiodermo, a brittle stor. Brittle 

 stars vary greatly in size, disc shape, arm length, and extent of arm 

 spines. Basket stars ore similar to brittle stars, but basket stars hove the 

 five arms much branched. 



echinoderms. Most can be found in fairly shallow 

 water, especially in cracks or crevices or under ob- 

 jects in tidepools. The one great problem in collect- 

 ing these animals is to obtain them with their arms 

 intact. Their practice of casting off parts or all of 

 one or more arms is defensive in that a predator is 

 likely to be attracted by the wiggling dismembered 

 portion of the animal. Castoff parts are regenerated. 



The greater mobility of these echinoderms is also 

 reflected by some ophiuroids being able to swim. 

 They use their arms much as a human swimmer does. 

 Their food consists of a variety of small, marine ani- 

 mals, especially crustaceans and mollusks, and some 

 feed on bottom debris. These stars in turn are 

 eaten by fishes. 



The sexes are usually separate, and fertilization 

 of the eggs either results in a free-swimming larval 

 stage or in development in parental brood pouches. 



QUESTIONABLE ENTEROCOELA: 

 CHAETOGNATHA 



Chaetognaths, the arrow worms, are another 

 subject of debate among zoologists. Differences of 

 opinion as to their classification exist because adult 

 arrow worms are pseudocoelomate and possess many 

 other resemblances to cavity worms. However, on 

 the basis of development, chaetognaths seem allied 

 to the enterocoelous phyla. This alliance, is at best, 

 remote, because arrow worms do not possess a larva, 

 their developmental enterocoel is not truly compa- 

 rable to that of unquestioned enterocoels, and it is 

 impossible to obtain true structural identity among 

 adult chaetognaths and other enterocoels. 



Assuming that Chaetognatha are Enterocoela, 

 there are three possible affinities for the phylum. 

 First, arrow worms may represent early divergence 

 from the ancestral enterocoel or from some archaic 

 animal on the supposed path from coelenterate to 

 enterocoel. That hypothesis is favored here. Sec- 

 ond, arrow worms may be a completely independent 

 line of divergence, again probably starting from 

 coelenterates. Finally, arrow worms may be an en- 

 tirely special case, perhaps a group representing 

 simplification from more complex ancestors. 



CHAETOGNATHA (Arrow Worms) 



Diagnosis: symmetry bilateral; unsegmented; 

 pseudocoelomate as adults; s inch to 2s inches long; 



