6 • CO. Whitmau 



cides with that of its host ; aud thus its limits are fixed , not by any 

 morphological boundary lines discoverable in the parasites themselves, 

 but indirectly through their assumed coineidence with the limits assigned 

 to a totally foreign geuus. 



To the genus Octopus corresponds the genus Dicyema\ to Ele- 

 done, Dicyemella ; to Sepia, Dicyemina ; and to Sepiola, Dicyemopsis. 

 Such is the facile method adopted by Van Beneden in carving out a 

 new system of generic names. Four genera were thus set up, and not 

 a single generic character pointed out ! If the correspondence affirmed 

 really existed , it would be a very remarkable fact , and , I believe, 

 quite an exceptional one : at all events , it would be preposterous to 

 suppose that it could be accepted as a fact without some substantial 

 evidence. I have to confess therefore to some surprise that van Bene- 

 den has neglected to offer any proof whatever in support of the position 

 he has taken ou this questiou, especially as on the assumed parallelism 

 hangs the validity of his whole system of Classification. The seven 

 species giveu in his paper are described with more or less detail ; but I 

 find no allusion to generic distinctions beyond what can be gathered 

 from the passage before cited. Presumably the descriptions of the 

 species include both specific and generic characters, but the line is no- 

 where distiuctly drawu between the two. 



Two distinct assertions are made in the above quotation : 



1 . Each Cephalopod has a single species of Dicyema. 



2. The species found in closely allied Cephalopoda are much more 

 nearly related than those found in Cephalopoda belonging to different 

 families. 



My observations warrant me in saying that neither of these asser- 

 tions is correct. 



In the descriptiou of species which is to follow the following facts 

 will be established : 



1 . Two Cephalopods , E. moschata and S. ofßcinalis , have each 

 two species of Dicyema. 



2. One species of Dicyema occurs in at least two different species 

 of Cephalopod , while another is found in at least three different Ceph- 

 alopods. 



3. Two species, B. Eledones "Wag. and D. gracile Wag., found in 

 Cephalopods belonging to two different families [E. moschata and S. 

 ofßcinalis] differ from each other less than they differ from the species 

 with which they are associated. 



4. In Rossia and Sepiola, two genera of the Myopsidae standing, 



