of the Bay of Naples. 109 



knowledge, and will, perhaps, very soon have to submit to 

 alterations. 



Characters of the Family Seisonidce. 



Elongate, vermiform animals, ^-3 millim. in length, of 

 similar form in both sexes, so that the sexual dimorphism is 

 expressed prominently only in the sexual apparatus. The 

 males are somewhat smaller than the females and not quite 

 so abundant. The hody is divided into four sections composed 

 of apparent segments, which are distinguished from before 

 backwards as head, neck, middle body (trunk), and tail, and, 

 witli the exception of the last two, are sharply separated from 

 each other. The neck can be retracted in its whole length 

 into the trunk, to the ventral surface of which it is then applied. 

 The rotatory apparatus is either present only in a rudimentary 

 form, or altogether wanting. The long, narrow (esophagus 

 opens into the anterior end of the masticatory apparatus, so 

 that the latter forms a sacciform, ventral appendage of the 

 oesophagus. In the head two dorsal and two ventrally situ- 

 ated, long-stalked, pyriform glands, which empty their secre- 

 tion before the mastax or into it. Muciparous cells of the 

 same kind exist also in the hind head and neck. Stomach 

 elongated, with two glands at the anterior end, formed of 

 large polygonal cells which bear no cilia within. Sexual 

 organs paired, but with a common dorsal evacuator, 

 which opens in the male at the point of passage of the neck 

 and trunk, in the female at the posterior extremity of the 

 trunk. The ovaries consist of numerous, distinctly separated 

 ova, and are therefore not differentiated into ovary and vitel- 

 ligene. The male sexual apparatus is of complicated struc- 

 ture, in which various parts, which are regarded as seminal 

 vesicle, vas deferens, and ductus ejaculatorius, may be distin- 

 guished. Two aquifh'ous vessels, furnished with " flicker- 

 organs " traverse head, neck, and trunk, and discharge them- 

 selves externally with the sexual organs. Characteristic of 

 these is (1) The development of some parts into thin-walled 

 and of others into thick-walled canals ; and (2) the absence 

 of a contractile vesicle. To the nervous system belongs a 

 ganglion placed dorsally in the head, and bearing a dorsal 

 feeler. There are no lateral feelers. The longitudinal mus- 

 culature is strongly, and that running transversely feebly 

 developed ; it is smooth throughout. The tail contains a 

 number of long-stalked, pyriform, viscous glands, which open 

 at the hindmost pole of the body. At the same point there is 

 towards the ventral surface a vesicle opening by a short pro- 



