122 ZOOLOGY 
side of the body, alternating with the diverticula of the ali- 
mentary canal (Fig. 77). They consist of sacs which arise in 
the dorso-ventral muscles (dissepiments), and are at first 
closed. The ova and mother-cells of the spermatozoa are prob- 
ably derived from the cells lining the walls of these sacs. 
When the reproductive cells are ripe, each sac opens to the 
exterior by a dorsally-placed pore. The eggs are often de- 
posited in mucous tubes secreted by the skin. Geonemertes 
palaensis, Tetrastemma hermaphroditica, and T. Kefersteinit, are 
hermaphrodite ; and Prosorhochmus Claparedit and Monopora 
vivipara bring forth their young alive. 
Among the ciliated ectodermal cells of many Nemertines, 
a number of unicellular glands occur; their secretion forms a 
copious mucus, which usually takes the form of a tube, in 
which the animal lives for a time, and which may be 
strengthened by grains of sand, ete. 
The opening of the mouth is beneath or behind the 
cerebral ganglia, and in Akrostoma, Malacobdella, and some 
others, the proboscis opens into the dorsal side of the mouth. 
The proboscis may be armed with rhabdites, and some observers 
have described nematocysts in Cerebratulus, etc.; these obser- 
vations, however, have not been confirmed. The morpho- 
logical nature of the proboscis and its sheath affords matter 
for much divergence of opinion. It is usually regarded as a 
developement of the anterior protrusible and retractile part of 
the body which occurs in the Turbellarian Proboscidea, 
Hubrecht, who has advocated a relationship between the 
NEMERTINES and the CHorDATA, regards the hypophysis cerebri 
of the latter as representing the proboscis, whilst the notochord 
represents its sheath. The latest writer on the subject, 
Biirger, lays stress upon the fact that the opening of the pro- 
boscis is never quite terminal, and on the relationship it holds 
to the mouth. He is inclined to regard the organ as a great 
developement of the Turbellarian pharynx, which has ceased to 
open into the alimentary canal, and has acquired a hollow 
sheath into which it can be withdrawn. 
In the Palaeo- and Schizo-nemertines the blood-vessels 
break up into a series of lacunar cavities in the head. In 
Drepanophorus the corpuscles are red with haemoglobin. 
