370 bulletin: museum of comparative zoology. 



of its young, partly because bachei has only very recently been figured 

 (Torrey, :04, pi. 1, fig. 1). 



Fortunately the Museum of Comparative Zoology contains a large 

 series of adult and young P. pileus, several specimens of rhodopsis 

 showing two stages in development, and 134 of bachei (one of them 

 belonging to Agassiz's original material). The "Albatross" speci- 

 mens, from the west coast of Mexico, are without question the latter, 

 and from the standpoint of geographic distribution the opportunity 

 to compare this form with the two Atlantic ones has proved instructive. 



I. Pleurobrachia pileus and P. rhodopsis. According to Chun ( : 80) 

 and to Moser ( : 03) the chief distinctions between these two are that 

 in the latter the ribs are very short, and that the junction of the 

 adradial canals with the meridionals is at the same level as the funnel. 

 But the material which I have examined shows that neither of these 

 distinctions holds good. One can hardly compare Chun's young 

 (5-7 mm.) rhodopsis ('80, taf. 2, figs. 5, 6) with A. Agassiz's drawing 

 of pileus at about the same stage ('65, p. 32, fig. 47) without being 

 struck by the resemblance between the two, and by the fact that the 

 ribs in the former are of about the same length as in the latter; but 

 in neither is the exact level of the junction of canals shown clearly. 

 Our smallest Mediterranean specimen is rather further advanced; 

 the ribs are proportionately longer, now reaching from near the apex 

 to about the mid level of the stomach; and the tentacular sheaths 

 and bases are larger. The specimen differs from Chun's figure in 

 having the opening of the sheaths nearer the aboral pole, while the 

 adradial canals open into the meridionals slightly aboral to the funnel, 

 instead of on a level with it. 



Unfortunately I have no corresponding stage of pileus for compari- 

 son, but this difficulty does not hold for our largest and most interest- 

 ing rhodopsis. This specimen though obviously somewhat shrunken 

 is 9 mm. long. Probably it was at least 10 mm. in life. The ribs are 

 much longer than in the younger specimen ; they now reach from close 

 to the aboral pole over § of the length of the stomach; the tentacle- 

 bases and -sheaths have increased in size, and the latter nearly equal 

 the oral ends of the ribs; and the junction of adradial and meridional 

 canals is appreciably above the level of the funnel. I was able to 

 compare this example with pileus, at a corresponding stage, and of 

 about the same size (9 mm.), and the only differences I could find 

 between them were that the funnel-canal was slightly longer in 

 pileus, the ribs slightly shorter. But in other specimens of pileus 

 only slightly larger the funnel-canal is shorter, and the ribs longer; 



