INVERTEBRATE EUCOEIOMATES 



261 



transparent, torpedo-shaped forms; heads with char- 

 acteristic bristles surrounding the mouth; body tube- 

 hke with lateral fins and tail terminated by a caudal 

 fin; free-swimming marine forms (Figure 15.6). 



About thirty species are known. They feed upon 

 microscopic organisms and live in the open ocean. 

 Because of their habitat, one is not likely to en- 

 counter them. Even if one were at sea, they would 

 be hard to find. They have a habit of being at depths 

 of several hundred feet during the day; however, at 

 night they do come to the surface. 



Arrow worms are hermaphroditic, but cross- 

 fertilization is the rule. Apparently true larval 

 stages do not occur; the eggs develop into a worm- 

 like nonswimming creature that looks much like the 

 adult. 



worms lack a larval stage make it difficult to establish 

 any relationship beyond reasonable doubt. 



HEMICHORDATA (Hemichordates) 



Diagnosis: symmetry bilateral; unsegmented, 

 with evidence of incipient segmentation; entero- 

 coelous; worm-like or vase-like; body of three regions, 

 proboscis, collar, and trunk; chordate-like in the pos- 

 session of gill slits and a dorsal hollow nerve cord in 

 the collar (but also a longer ventral one), but without 

 a particular supporting rod (notochord) beneath the 

 dorsal nerve cord; have a similar appearing rod (in 

 the posterior proboscis), as an anterior outpocketing 

 of the mouth cavity rather than the primitive gut 

 proper as in the notochord; all marine. 



Figure 15.6 Sagiiia, an arrow worm (adults mostly 12 to 25 mm. 

 long, but ronge from 5 to 140 mm. long). (Redrawn from The Encyclo- 

 pedia of the Biological Sciences, Peter Gray, ed. Rernhold Publishing 

 Corp., New York, 1961.) 



BILATERAL, NONCHORDATE 



ENTEROCOELA: HEMICHORDATA 



AND POGONOPHORA 



The echinoderms, hemichordates, and chordates 

 are considered to be closely related. In fact, hemi- 

 chordates are seemingly sufficiently allied for some 

 zoologists to treat them as a subphylum or class of 

 the Chordata. Also, many zoologists unite the phyla 

 Echinodermata, Hemichordata, and Chordata in a 

 Superphylum Echinodermata. The Chaetognatha 

 may also belong to this major taxon. 



The Superphylum Echinodermata may encompass 

 another group, the Pogonophora (beard worms). 

 This taxonomic arrangement is made because beard 

 worms seem closely related to hemichordates, 

 either by having evolved from definite hemichordates 

 or, more likely, having shared an ancestor that could 

 be considered neither hemichordate nor beard worm. 

 These assumptions are based on the many apparent 

 fundamental similarities in structure. However, 

 there are difficulties in joining the two phyla. Beard 

 worms have neither gill slits nor a digestive tract. 

 These missing structures and the fact that beard 



CLASS ENTEROPNEUSTA (Acorn Worms) 



Diagnosis: body worm-like, without a stalk, 1 to 

 100 inches long, mostly 5 to 25 inches long, fleshy 

 and contractile; proboscis (prostome) cylindrical but 

 drawn to a blunt point, short or long, somewhat 

 tongue- or acorn-like; collar cylindrical, usually as 

 wide as long; trunk elongate, with numerous paired 

 gill slits entering pharynx; digestive tract straight; 

 solitary, mostly intertidal animals that either form 

 burrows or live under rocks, in rock cavities, or 

 among algae, but also at some depth; filter feed by 

 burrowing with mouth open; when water, sand, mud, 

 and organic matter reach pharynx, water is expelled; 

 after organic matter is digested and assimilated, 

 sand, mud and undigested organic matter is expelled 

 through the anus; sexes separate; a larva resembling 

 that of certain echinoderms may be produced 

 (Figure 15.7). 



CLASS PTEROBRANCHIA (Pterobranchs) 



Diagnosis: body vase-like, with a stalk, to 5 inch 

 long; proboscis shield-shaped; collar cylindrical, 

 usually short, bearing 1 or more pairs of tentacles 

 (lophophore); trunk relatively short, with one or no 

 pairs of gills; digestive tract U-shaped with anus near 

 mouth; sedentary, solitary, and tubeless or aggre- 

 gated or colonial and forming tubes that are perma- 

 nently fixed to substrate; found from shallow seas to 

 deeps; filter feeders using lophophore; hermaphrodi- 

 tic or with separate sexes; larval stages known in 



