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PROCEEDINGS OF THE 



[Dec, 1857. 



sort. From the upper extremities on the other hand, passed 

 upward to the sense-capsule, (which presented no peculiarity hut 

 that of being proportionately large,) from each of the ambulacra, 

 a whitish line corresponding exactly in position with the delicate 

 tubes traced by Agassiz between the same points in Pleurobra- 

 chia. 



To return to the large lateral sinuses, it will be noticed in the 

 drawings (fT. 3 and 4,) that they were each somewhat quadran- 

 gular in form, one of the angles being turned towards the apex of 

 the digestive cavity, one to each pair of ambulacra, and one to a 

 structure not yet mentioned — the chamber of the tentaculum. In 

 fig. 4, however, there is, as yet, no angular projection correspond- 

 ing to the tentaculum, which is sessile upon one side of the 

 triangular sinus. Each of the angles belonging to the ambulacra, 

 gave off two very short projections, already mentioned : one of 

 which was distributed to each ambulacrum. The fourth, which 

 is situate between the ambulacral angles, on reaching the peri- 

 phery, meets, and is partially embraced by the external depres- 

 sion which forms the tentacular chamber. This chamber increases 

 in diameter as we trace it outwardly, and encloses a short tentacu- 

 lum, with three or four nodose threads. The tentaculum was 

 thus on a level, that is, in the same horizontal plane with the 

 lateral sinuses, as is more or less the case in the species of 

 Cydippe and Pleurobrachia. 



Immediately above the apex of the digestive cavity, and at the 

 point of junction of the two lateral sinuses, arose the ascending 

 diverticulum of the circulatory system, which, as in adult Beroids, 

 embraced the base of the sense capsule. The size of this structure 

 rendered it very conspicuous. The dark nucleus within, consisted 

 of an aggregation of spherical corpuscles of a blackish hue, 

 which appeared to me explicable, on the supposition of high 

 refractive power, and not as necessarily indicating pigmentary 

 deposit. These corpuscles appeared to be not more than twelve 

 or thirteen in number. The nucleus thus formed was covered by 

 a very thick transparent cap, of an oblong form, having its greatest 

 diameter in the vertical diameter of the body. 



In this young stage the animal was quite lively, the cilia being 

 in constant motion, though on account of their small number, its 

 progress through the water was not rapid. The un vascular por- 

 tion of the pyriform body, between the plane of the circulatory 

 system and the mouth, was quite flexible, and was turned about in 



