ANATOMY AND PHYSIOLOGY OF SALPA AND PYROSOMA 4I 



tremity than at the anterior {a) ; the former ends in a point, the latter 

 in a small square facet. 



The sides are flattened into several irregular facets, and the upper 

 and lower edges are sometimes somewhat carinated. The apertures 

 are similar in general structure to those of the form A, and the 

 anterior and posterior extremities project considerably beyond them. 



The ganglion {d) is placed at about one-fourth of the length of 

 the body from the anterior extremity. The otolithes resembled 

 those of A. 



The endostyle (c) is not nearly equal in length to half the body ; 

 it does not extend so far back as to the third muscular band. 



The outer surface of the integument is smooth. 



The muscular bands {k) are five in number, and none of them 

 encircle the body of the animal, the dorsal extremities being always 

 separated by a considerable interval. This form, when young, was 

 sometimes found in chains ; the adults were always separate. 



These forms, it will be observed, are widely different, and the 

 difference is as great between the youngest forms of each as between 

 the adults, so that they are not derived from one another by any 

 species of metamorphosis, properly so called. 



Whatever be their external differences, however, their internal 

 organization is so similar that the same description applies to both. 



9. The Salpa, then, may be considered as a hollow cylinder, 

 consisting of two tunics, an external and an internal (a, /?), the 

 former (a) forming the mantle, the latter (^) the wall of the respira- 

 tory cavity. These tunics are continuous with one another at the 

 respiratory apertures, but elsewhere they are separated by a more 

 or less wide space. 



In very young Salpce this space is like the cavity of a serous sac, 

 but in the older forms it becomes broken up into smaller channels by 

 the adhesion of the inner and outer tunics to one another at various 

 places, and so constitutes a system of sinuses ; it may be conveniently 

 called the " sinus system." 



10. Running obliquely from behind forwards and downwards, a 

 thickish column or band {e) crosses the respiratory cavity ; it is 

 hollow, and its cavity opens above and below into the sinus system. 

 This is the "gill." 



It presents an edge anteriorly and superiorly, and on each side 

 of this, the lateral surfaces are beset with a series of small, oval, 

 ciliated spaces. In this species the gill has but a single grand sinus 

 running through it, and presents no appearance of vascular ramifica- 

 tions. The name gill has been applied to this structure .somewhat 



