164 ZOOLOGY. 



the specific and generic differences being very slight. They 

 have a mouth and digestive canal (except in Echinorhynclius), 

 the integument being hard, chitinous, and not segmented 

 (except in Desmoscolex, which approaches in this respect the 

 annelids), and usually smooth, except in Echinoderes, which 

 is variously armed with hair-like spines. Each end of the 

 body is much alike, the mouth situated at the anterior end, 

 and the anal opening at or near the conical tip of the body. 

 There are two long vessels which extend from a single com- 

 mon pore situated on the median line of the under side of the 

 body, a short distance from the head ; these are supposed to 

 be excretory vessels. In. Ascaris and Oxyuris a nervous ring 

 surrounds the oesophagus, from which two nervous threads, 

 one dorsal the other ventral, pass to the end of the body, and 

 there are six other smaller longitudinal nerves. The gangli- 

 onic cells lie near the nervous ring, forming a suboesopha- 

 geal, supracesophageal and lateral ganglion, and there is also 

 a caudal ganglion. In some free-living ISTematodes there are 

 eye-specks. 



The Nematodes are usually bisexual ; Pelodytes is her- 

 maphroditic, while the same individual of Ascaris nigrovcnosa 

 at first produces sperm-cells and afterwards eggs. The males 

 differ from the females in their smaller size and the usually 

 curved end of the body. While most of thes: worms lay 

 eggs, some, as in Trichina spiralis, bring forth their young 

 alive. 



The mode of development of these true Nematode worms 

 {Echinorhynchus excepted) so far as known is quite uniform, 

 growth being direct, without any metamorphosis. The 

 germ is formed in three ways : (1) usually the egg under- 

 goes total segmentation ; (2) others, as in Ascaris dcntata 

 and Oxyuris ambigua, do not show any apparent trace of seg- 

 mentation, while (3) in C^icuUanus elegans there is no yolk, 

 the nucleus absorbing all the vitelline matter, which is lim- 

 pid and transparent. The germ consists of a single series or 

 circle of cells bent on itself, somewhat as in Fig. 120, which 

 represents a little more advanced stage in Sagitta, and there 

 are a few cells representing the endoderm. The embryo 

 rapidly assumes the adult form before hatching. 



