204 
UNSEGMENTED “WORMS.” 
unite anteriorly, and open in a ventral excretory pore near the head. 
They seem to be associated internally with fixed phagocytic cells. 
In the species discussed there are four giant branched cells situated 
PY, 
Fic. 107.—Diagram of 
the structure of a 
male Nematode. 
M.,mouth; G.,cesophagus; 
GA., nerve ring; B., bulb 
atlowerend of fore-gut; G., 
mesenteron; S/., spine 
with sheath; 4., anus; 
D., ejaculatory duct; VS., 
seminal vesicle; 7., testis; 
ET,, longitudinal excre- 
tory tube, cut short; 2P., 
excretory pore. 
anteriorly, which are especially connected with 
taking up waste particles. The relation of 
this excretory system to that of other In- 
vertebrates is unknown. 
The sexes are separate. In the 
male the testis is unpaired—a coiled 
tube gradually differentiating into vas 
deferens, seminal vesicle, and ejacu- 
latory duct. The genital aperture is 
close to the anus. The spermatozoa 
have not the typical form, and are 
sluggish. In the female the ovary is a 
paired tube, which passes gradually into 
an oviduct and a uterus at each side, 
and a short unpaired vagina. The 
genital aperture is ventral and anterior. 
The ova meet the spermatozoa at 
the junction of uterus and oviduct. 
Segmentation is total, and results in 
the formation first of a blastula and 
then of a gastrula. The germ-cells are 
distinguishable very early from the 
body-cells. Blastopore and archen- 
teron are obliterated, the mid-gut 
arising as a secondary splitting between 
two rows of endoderm cells. The eggs 
pass out of the gut of the host and 
probably hatch in water, and are thus 
re-introduced. No intermediate host 
has yet been found. 
The Nematoda form an important group, 
interesting both on account of their parasitism 
and on account of their peculiarly isolated 
zoological position. Though parasitism is 
exceedingly common, many are free - living 
for at least a part of the life cycle, and feed on putrefying organic 
matter. 
Again, although the number of individuals which may infest 
‘one host shows how successful the parasitism is, yet Nematodes exhibit 
few of the ordinary adaptations to a parasitic life, and there is no sharp 
structural line of demarcation between free and parasitic forms, Among 
