182 INVERTEBRATE MORPHOLOGY. 



of separated ova. They pass to the exterior by a complicated 

 system of ducts, the most anterior portion of which is a wide 

 funnel-shaped structure, the hell, to whose wall the ligament is 

 attached and which, by a rhythmical expansion and contrac- 

 tion, engulfs the ova and ova-masses floating about in the 

 coelom. From the lower end of the bell they escape, the ova- 

 masses to be returned to the coelom, while the fertilized sepa- 

 rate ova pass into a short tube, the oviduct, which opens below 

 into a muscular uterus, which finally communicates with the 

 exterior at the posterior end of the body. 



The male apparatus consists of usually two testes (Fig. 

 90, t) enclosed within the ligament, which is attached below 

 to the wall of the evertible bursa. From each testis a duct 

 passes backwards, the two soon uniting to the single vas de- 

 ferens, which, after receiving the ducts of some unicellular 

 glands {gl), opens into the bursa at the tip of a muscular pe?iw 

 (p). The bursa when everted is a somewhat funnel-shaped 

 structure at the bottom of which is the penis, the edge being 

 furnished in some forms with hooks by means of which it 

 serves as a copulatory organ. 



The life-history of the Acantliooephala includes a change of host. The 

 larvae are found in the body-cavity of Crustacea or insects, and reach ma- 

 turity only when the intermediate hosts are swallowed by the proper final 

 host. The largest species of EcMnorhynchus is the E. gigas, which occurs 

 in the intestine of the pig ; the intermediate host of this form is the June 

 bug {Melolontha). 



Nothing can as yet be stated with any certainty concerning the relation- 

 ships of the Acantliooephala. They are usually associated with the Nema- 

 todes, to which they certainly present similarities, but no intermediate 

 forms bridging the gap between the two classes are yet known, and the 

 embryological history throws little light upon the question. 



SUBKINGDOM METAZOA. 

 TYPE NEMA THELMINTHE8. 



I. Class Nematoda.— With digestive tract ; without proboscis furnished 

 with chitinous hooks. 

 1. Order Eimematoda. — Musculature of body-wall interrupted along 

 the lateral line ; no mesentery ; no peritoneal epithelium. An- 

 guiUidu, Tylenclius, Heterodera, Trichina, Ascaris, Oxyuris, 

 Trichocephalus, Doohmitis, Filaria. 



