290 



COELHELMINTHES 



In development the brachiopods recall both Sagitta and the Annelida. 

 They resemble Sagitta in that in Argiopc the ccelom arises by outgrowths from 

 the archenteron (fig. 279), divided later by septa into three pairs of pouches. 

 They are annelid-like in the form of larva and in the presence of chaetse which 

 are formed in separate follicles. Brachiopods were formerly so numerous that 

 they are among the most important fossils in the determination of geologic 

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

 of the sea. 



FIG. 279. Development of brachiopod (after Kowalevsky). A, gastrula with 

 early enteroccelic pouches; B, closure of blastopore; C, ccelom. separated, body annu- 

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

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

 d, dorsal lobe of mantle; e, enteroccele; m, mantle; v, ventral mantle lobe. 



IG ' 



Order I. Ecardines. 



Hinge absent: valves similar, the stalk passing between them (Lingula*), 



or unequal, the ventral perforated by the stalk (Dis- 

 cina), or the animal is directly attached by the shell 

 (Crania). 



Order II. Testicardines. 



Hinge present, valves unequal, the ventral per- 

 forated by the stalk; anus degenerate. Rhyn- 

 chonclla* Terebratulina* in our colder waters. 

 Spirifer, Orthis, Pentamenis, A try pa, important fossil 





genera. 



Summary of Important Facts. 



(1) The CCELHELMINTHES have a well-developed body cavity (ccelom). 



(2) The CEUETOGNATHI are hermaphroditic worms with three pairs of 

 coelomic pouches, with fins, a'-d bristle-like jaws. 



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

 with cylindrical unsegmented body, with nerve ring (no ganglia), paired excre- 

 tory organs, and tubular gonads. 



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

 in the small intestine, O.vwm vcrmic-ularis in the large intestine, Ankylostoma 

 duodenalis, and the notorious Trichina spiralis. In hot climates Filaria san- 

 gulnis ho ni in is and Dracunculus medinensis. 



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

 are parasitic in insects. 



