16 BUSK, ON THE STRUCTURE AND 



The present account of the animal, however, has been com- 

 piled chiefly from the observations of Krohn and Wilms, 

 whose papers on the subject appear to include nearly all of 

 importance that has as yet been made out respecting the 

 anatomy and physiology of Sagitta. 



The body of the Sagitta Lijmnctata is as transparent and 

 clear as glass, cylindrical or slightly flattened, pretty regu- 

 larly fusiform, though rather more attenuated posteriorly than 

 in front (PI. II. fig. 1), when it again expands at the extremity. 

 It is divided into three distinct portions, the " head,' " trunk,'' 

 and " caudal portion," which are separated from each other 

 by transverse septa. Each of these portions will be sepa- 

 rately described. Posteriorly the body is furnished, on the 

 sides and extremity, with five delicate membranous expansions, 

 which have received the name of " fins," though bearing no 

 real analogy with the fins of a fish. These " fins " are all 

 in the same plane, and spring from a line equidistant between 

 the dorsal and ventral surfaces. The anterior pair of lateral 

 fins, which are far smaller than the posterior, are situated 

 nearly in the middle of the body, being equal in length to 

 about l-5th of the extreme length of the animal. The 

 posterior pair of lateral fins, which are both longer and wider, 

 extend from the posterior border of the former to within a 

 short distance of the caudal extremity, where they terminate 

 rather abruptly. Anteriorly the two pairs of fins are often 

 apparently continuous with each other by a very narrow band 

 of similar texture ; so that, in fact, in many cases the lateral 

 fins might be described as constituting only one pair, of varying 

 width in different parts. The caudal fin is, however, quite 

 distinct. It is broad and somewhat rounded, expanding like 

 a fan from the posterior extremity of the body, and passing a 

 short distance up on each side. These " fins" are composed 

 of an excessively delicate and apparently structureless mem- 

 brane, which is strengthened by very slender radiating fibres, 

 placed very closely together, and appearing to be somewhat 

 thicker at the base than more outwardly. Although very 

 slight injury tears the fin in the direction of these apparent 

 fibres — and its edge, thence, often appears to be fimbriated — 

 the fibres themselves cannot be readily isolated, and there is 

 every reason to believe that the edge of the fin in the perfect 

 state is entire. 



The integument, except on the head, is comparatively 

 speaking thick and dense. It is covered with a very delicate 

 epidermis, composed of rounded or polygonal cells. The 

 existence of this epidermis was denied by Krohn in his first 



