328 



C(ELUELMINTHES. 



Now there are but few species, some inhabitants of the greatest depths of 

 the sea. 



FIG. 301. Development of brachiopod. (After Kowalevsky.) A, gastrula with early 

 enterocrelic pouches; B, closure of blastopore; C, coelom separated, body annu- 

 lated; D, cephalic disc and mantle developing, the latter with long setae; E, at- 

 tached embryo, the mantle lobes folded over cephalic disc (setae omitted), c, 

 cephalic disc; d, dorsal lobe of mantle; e, enterocoele; rn, mantle; v, ventral man- 

 tle lobe. 



Order I. Ecar dines. 



Hinge absent; valves similar when the stalk passes out between them 

 (Lingula *), or unequal when the ventral is perfo- 

 rated by the stalk (Discina) or when the animal is 

 directly attached by the shell (Crania}. 



Order II. Testicardines. 



Hinge present, valves unequal, the ventral 

 perforated by the stalk; anus degenerate. Rhyn- 

 chonella,* Terebratulina* in onr colder waters. 

 Qpirtfer, Orthis, Pentamerus, Atrypa, important 

 fossil genera. 



FIG. wrebratuiia sep 

 tentrionalig* 



Summary of Important Facts. 



(1) The CCELHELMINTHES are characterized by a well-developed body 

 cavity (coelom). 



(2) The CH.ETOGNATHI are hermaphroditic worms with three pairs of 

 ccelemic pouches, with fins, and bristle-like jaws. 



(3) The NEMATODA are mostly dioecious, usually parasitic elongate 

 worms, with cylindrical unsegmented body, with nerve ring (no ganglia), 

 paired excretory organs, and tubular gonads. 



(4) The most important species parasitic in man are Ascaris lumbri- 

 coides in the small intestine, Oxyuris vermicularis in the large intestine, 

 the blood-sucking Ankylostoma duodenalis, and the notorious Trichina 

 spiralis. In hot climates occur Filaria sanguinis Jwminis and Dracun- 

 culns medinensis. 



(5) The GORDIACEA have mesenteries and splanchnic mesoderm; they 

 are parasitic in insects. 



(6) The ACANTHOCEPHALI lack alimentary tract, have a spiny proboscis 

 and a very complicated reproductive apparatus. The adults are parasitic 

 in vertebrates, the young in arthropods. 



