NO. 1989. ASCIDIAN8 FROM NORTHEASTERN PACIFIC— RITTER. 469 



In the length and vokiminousness of the digestive canal this species 

 is entirely unique among ascicUans, so far as I know. In a specimen 

 80 mm. long the intestine, including the stomach, was 187 mm. long, 

 or one and a tliird times the length of the animal. The intestine, 

 exclusive of the stomach, was 142 mm. long in this individual. 

 The gut of a dissected animal resembles that of a mouse quite as 

 much as it does that of an ordinary ascidian. 



So far as I have been able to determine the canal of Styela nord- 

 enskjoldi Michaelsen, 1900, from the region of Cape Horn, approaches 

 that of tliis species more closely than any other. However, there is 

 no such elaborate coiling of the organ in the South American species 

 as in S. Tnacreriteron, and there is little in common between the two 

 species in other respects. 



Many of the individuals possess a stout, hard, sometimes quite 

 regularly pyramidal spike or horn between the siphons (fig. 18). In 

 this the species strongly resembles a Styela that has been known as 

 ;S'. monoceros because of tliis structure. There is, however, no possi- 

 bihty of identifying the animals before us with the S. monoceros of 

 the north European seas. Herdman (1893), who considers monoceros 

 to be a good species, has described and figured the ajiimal, and the 

 simplicity of the intestine as shown by him would of itself be nearly 

 conclusive as to the specific distinctness of the two creatures. But 

 as a matter of fact they differ to some extent in almost every respect, 

 macrenteron being somewhat larger in size and more cyhndric in form; 

 having a larger number of tentacles, more internal vessels on the 

 branchial sac, both on the folds and between them; more stigmata 

 between the vessels, and, seemingly, a larger number of folds in the 

 stomach waU. Hartmeyer, 1899 and 1903, has returned to the older 

 view that monoceros is only a form of S. rustica, there occurring, ac- 

 corcUng to liim, homed individuals of rustica which are indistin- 

 guishable, specifically, in any other way from hornless animals. 



Not having had an opportunity to study the subject first hand in 

 any critical way, I would not presume to pass upon the question of 

 the specific distinctness of monoceros. At the same time it is worth 

 while to point out that the presence of a horn very similar indeed to 

 that present in monoceros, in most, though not in aU, individuals of 

 another very distinct species, shows the somewhat sporadic character 

 of the structure, and so indicates its unrehabiHty as a species mark. 

 And it should be noted further that in most individuals of S. macren- 

 teron there is a marked tendency to a tuberculation of the test around 

 and between the siphons, even when the horn proper is doubtfully 

 present, thus suggesting that the horn should be looked upon as 

 one of the test tubercles specially well developed in most but not 

 in all individuals. 



