150 THE VOYAGE OF H.M.S. CHALLENGER. 



Although Savigny in his " Memoires " introduced the name Styela, Macleay was, as 



far as I am aware, the first to use it as a generic term. This was in 1823, but the genus 

 was not accepted by subsequent writers, and the species of Styela were included in Cynthia 

 until 1868, when Hancock made use of the word " Styela" in a generic sense, and 1874, 

 when Heller returned to the original spelling, and defined the genus Styela as it is 

 employed here. Kupffer (" Jahresber. der Commiss., &c," 1875) does not recognise any 

 of the sub-divisions of Cynthia, and Traustedt accepts Styela, not in Heller's sense, but 

 as including Polycarpa. 



Styela is distinguished from Pelonaia and Baihyoncus very clearly by the struc- 

 ture of the branchial sac. It is more difficult to separate it from Polycarpa, but as I 

 have said above (page 149), it is convenient to give generic names to the two groups of 

 species, until we find a connecting link. Heller gave two characteristics by which they 

 differ, — (1), the course of the intestine, which in Styela forms a narrow loop, while in Poly- 

 carpa it takes a wide, open curve ; and (2), the genitalia, which are found in Styela as one, 

 two or more, but never many, long simple or branched organs, while in Polycarpa they 

 form a large number of generally small and rounded bodies scattered over the inside of 

 the mantle upon both sides of the body, and called " polycarps " by Heller. 



dl. 



Fig. 17. — Diagrammatic transvers section of the Branchial Sac of Slyel". 

 I., II., III., IV. br.f., the branchial folds ; d.l., the dorsal lamina ; en., the eudostyle ; mh., one of the meshes. 



The branchial sac is found in various conditions in the genus Styela. In typical forms 

 it has eight well-marked folds, four upon each side (fig. 17, I., II., III., IV". br.f.), and 

 a greater number is never present ; but many species have less than eight. The fold 

 nearest to the endostyle on each side appears first to become slighter, less of a true fold, 

 and more of a mere crowding together of the internal longitudinal bars, and then finally 

 disappears. In Styela grossularia there is only one recognisable fold in the branchial 



