No, 6. — Studies in Morpliogcncsis. — I. On the Development of the 

 Cerata in jEoHs. By C. ]>. IJAVENrouT.^ 



While at Mr. Agassiz's Newport Laboratory last summer, I collected, 

 early in August, some specimens of /Eolis^ which occurred abundantly 

 on the Hydroids. Tlie individuals varied greatly in size, and, since Nu- 

 dibranch eggs had been abundant on the same Hydroids earlier in the 

 season, I regarded most of them as inmiature. 



I was particularly struck by the regularity with which the cerata, or 

 dorsal papillee, were distril)uted over the surface of the body, and the 

 constancy in position of young cerata in relation to the older ones. 



Figures 1 and 2 are drawings of t<v'o individuals showing the arrange- 

 ment of the cerata. They are placed in transverse rows, which are not 

 equidistant, however, but grouped in twos or threes. In each transverse 

 row the largest ceras lies dorsalmost, the smaller more ventral, the 

 youngest of all being often indicated by only a slight protuberance at 

 the base of and ventral to the next older. The regularity of the pro- 

 cess of budding of new cerata induced me to make sections to determine 

 its details. 



The cerata of ^olis contain, as is well known, processes of the ali- 

 mentary tract, — the so called hepatic coeca (Plate II. Fig. 12). They 

 are covered externally by the ectodermal epithelium. Between ectoderm 

 and hepatic ccecum are mesodermal cells, which lie (1) in a thin layer at 

 the inner surftice of the ectoderm, (2) in a thin layer over the coecum, 

 and (3) between these two layers, surrounding blood spaces or loosely 

 distributed. 



The coecum of each ceras connects at the base with a lateral diverticu- 

 lum of the alimentary tract, — gastric diverticulum, — with which also 

 the coeca of all of the other cerata of the same transverse row unite 

 (Plate I. Fig. 3, ga. dv.). There are as many of these gastric diverticula 

 as there are transverse rows of cerata (Fig. 4, ga. dv.). 



1 Contributions from the Zoiiiogical Laboratory of the Museum of Comparative 

 Zoillogy, under the direction of E. L. Mark, No. XXXVII. 



- Owing to the immaturity of tlie individuals, I was unable to determine the 

 species accurately. Figures 1 and 2, however, show the external form of the 

 speeies (one or two) employed. 



VOL. XXIV. — NO. 6. 



