OBSERVATIONS ON THE GENUS SAGITTA 97 



of the mouth. But on the other hand, it differs widely from them in 

 the nervous system, the sexual system, and the nature of the muscular 

 tissue. 



2. Sagitta has no small resemblance to certain Naiadje, in which 

 when }'oung the anterior hook-like feet are directed forwards parallel 

 to the mouth. It differs from them in the nature of its nervous 

 system, which exhibits a concentration quite foreign to the annelid 

 type, in the nature of the muscular tissue, and in the total absence of 

 any water vascular system. 



3. and 4. The real affinities of Sagitta are probably with one or 

 other of these great divisions. The structure of the nervous and 

 muscular system speaks strongly for this view, and the nature of the 

 sexual system is not opposed to it, inasmuch as we have hermaphrod- 

 ism among both the lowest Crustacea (Cirrhipedia) and the lowest 

 Arachnida (Tardigrada). 



The study of development can alone decide to which of these 

 divisions Sagitta belongs ; but until such study shall have demon- 

 strated the contrary, Mr. Huxley stated his belief that Sagitta bears 

 the same relation to the Tardigrada and Acaridse, that Linguatula 

 (as has been shown by Van Beneden) bears to the genus Anchorella, 

 and that the young Sagitta will therefore very possibly be found to 

 resemble one of the Tardigrada, the rudimentary feet with their hooks 

 being subsequently thrown up to the region of the head, as they are 

 in Linguatula. 



VOL. I f^ 



