122 ZOOLOGY. 
the specific and generic differences being very slight. They 
have a mouth and digestive canal (except in Hchinorhynchus), 
the integument being hard, chitinous, and not segmented 
(except in Desmoscolez, which approaches in this respect the 
annelids), and usually smooth, except in Hchinoderes, 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 subcesopha- 
geal, supracesophageal and lateral ganglion, and there is also 
a caudal ganglion. In some free-living Nematodes there are 
eye-specks. 
The Nematodes are usually bisexual; Pelodytes is her- 
maphroditic, while the same individual of Ascaris nigrovenosa 
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 dentata 
and Ozyuris ambigua, do not show any apparent trace of seg- 
mentation, while (3) in Cucullanus elegans there is no yolk, 
the nucleus absorbing all the vitelline matter, which is lim- 
pid and transparent. The germ consists of asingle 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. 
