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 

 palaen^is, Tetrastemma hermaphroditica, and T. Kefersteinii, are 

 hermaphrodite ; and Frosorhochmus Claparedii 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, etc. 



The opening of the mouth is beneath or behind the 

 cerebral ganglia, and in Akrostoma, Malacohdella, 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 Cerehratulus, 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 TurbeUarian Prohoscidea. 

 Hubrecht, who has advocated a relationship between the 

 Nemeetines and the Choedata, 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 incUned to regard the organ as a great 

 developement of the TurbeUarian 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 sheath into which it can be withdrawn.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 

 DrepaTwphoriis the corpuscles are red with haemoglobin. 



