428 PHYSIOLOGY OF THE INVERTEBRATA. 



in which the Neniatoidea are sexless worms. In the genera 

 Mermis and Gordius, the anterior ends of the embryos are 

 provided with spines — the spines being used to pierce holes 

 in the bodies of insects, in which these Nematodes live a 

 portion of their life-history. According to Sir John Lubbock, 

 the male of Sphcerularia becomes permanently fixed to the 

 female. Many of the Nematodes are only parasitic in the 

 sexless stage of their existence ; but some are free in the 

 larval or sexless stage ; and some again are parasitic both in 

 the sexless and sexual condition. 



In the AcanthocepJiala — represented by JEchinorhi/iichus — 

 the genital organs are attached to the posterior end of the 

 proboscis sheath by the ligamentum suspensorium, which 

 traverses the body cavity. The sexes are distinct. The 

 female organ consists of a tubular ovary ; a uterine bell — the 

 mouth of which opens into the body cavity, while the upper 

 portion leads into the uterus; the uterus then passes 

 into the vagina, which opens externally at the posterior 

 end of the body. The ova, discharged from the ovary into 

 the body cavity, are ultimately taken up by the mouth of the 

 uterine bell, which is continually expanding and contracting, 

 and thence the ova pass into the uterus. The male organ 

 consists of two oval testes provided with vasa deferentia, which 

 proceed to the posterior end of the body. At this point the 

 vasa deferentia fuse together, forming a bulb-like structure, 

 called the ductus ejaculatorius, which is usually provided 

 with six or eight glandiilar sacs. The ductus ejaculatorius is 

 furnished with a long penis placed in the centre of the 

 bottom of a bell-shaped bursa situated at the posterior end of 

 the body. The development of the fecundated ovum com- 

 mences with an irregular and a complete segmentation. This 

 gives rise to an embryo, which is enclosed in the membranes. 

 The embryo, at this stage, is provided anteriorly with 

 small spines. The ova (containing the embryo) are usually 

 "swallowed" by various Avvphipoda, Isopoda, and Inseda 

 (larva3) ; in this stage the membranes are dissolved by the 



