ON DINOPHILUS GIGAS. 265 



pletely fill the body cavity, the alimentary canal becomes much 

 reduced in size, and it and the ectoderm appear to undergo a 

 kind of fatty degeneration. I could find no ducts of any kind 

 for the generative products, and from the condition of the 

 tissues of ripe individuals, I have no doubt that, when the 

 generative products are mature, the animals rupture their body 

 wall and die. If this be true, it explains the sudden disap- 

 pearance of Dinophilus at the end of spring, which has been 

 noticed by Hallez^ and others. In the case of D. gigas, 

 all the individuals collected at Mount's Bay on April 22nd had 

 undergone so much degeneration that they were quite useless 

 for histological purposes, while the absolute number of indi- 

 viduals collected between the 16th and 23rd of April was so 

 small compared with the number obtained in the same time a 

 fortnight earlier, as to show that the process of disappearance 

 was beginning. 



III. — On the Systematic Position of Dinophilus. 



It is hardly necessary to indicate the points of resemblance 

 between Dinophilus and a fairly late Chsetopod larva. The 

 ' ciliated rings and the ventral plate of ciliated ectoderm, asso- 

 ciated with a pair of unsegmented lateral nerve-cords; the ciliated 

 alimentary canal, with its large stomach, its narrow oesophagus 

 with a muscular pharynx, and its intestine ; these are features 

 in which all species agree with a late Polygordius larva, while 

 in D. gyrociliatus Ed. Meyer finds that the excretory sys- 

 tem is "almost identical with that of a Nereis larva." ^ The only 

 point of difference between Dinophilus and the Archiannelids 

 is the absence of an epithelial body cavity, and this character, 

 in spite of the importance given to it by many observers, 

 seems to be, in this case at least, of secondary importance. 

 For in the first place the body cavity of Saccocirrus seems to 

 be devoid of any definite epithelium ;* while in the second place 



1 'Histoire naturelle des Turbellaries,' Lille, 1879. 



2 Quoted by Lang, ' Monographie der Polycladen,' p. 679. 



' Compare the figures given by Fraipont, ' Archives de Biologie,' Tome v, 

 PI. xiv, which are confirmed by sections in the Cambridge Laboratory. 



