194 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



apparent that its development took place wholly in the free-swimming 

 stages. A transverse section through the antennule of a newly hatched 

 larva (Plate 4, Fig. 19) shows no sign of invagination in the region 

 wliere the sac is to appear. But certain elongated nuclei, evidently 

 those of modified hypodermal cells, are found grouped, two or three 

 layers deep, beneath the dorso-lateral wall of the appendage (Fig. 19, 

 cl. ma.). These elongated nuclei, viewed from the dorsal surface of the 

 appendage, are seen to be roughly arranged in a semi-circle, like the rows 

 of otocyst hairs in Figure 26 (Plate 5), and when traced through later 

 stages, the position they occupy is found to be directly beneath the ridge 

 where the sensory hairs later appear (Plate 5, Fig. 24, set. ot.). They 

 are evidently, therefore, the nuclei of the matrix cells which build up 

 by secretion the chitiuous walls of the sensory hairs. These cells, like 

 those which take part in hair formation after ecdysis, originate from the 

 chitinogenous hypodermal cells by simply becoming elongated and sink- 

 ing beneath them. A similar arrangement of matrix cells was found in 

 the developing otocyst of Mysis by Bethe (95*). Numerous spherical 

 nuclei, which stain in a manner characteristic of nerve cells, are present 

 just below the matrix cells (Fig. 19, n'bl.). If traced back to the gan- 

 glionic masses of the brain, they are found to be continuous with the 

 nerve cells of the latter, and probably originate from them. 



b. Second Larval Stage. 



(Second to fifth pair of abdominal appendages present.) 

 In this larva the first evidence of invagination is seen on the dorsal 

 side at the base of the antennule (Plate 4, Fig. 20). The nuclei of the 

 matrix cells are now larger, and very conspicuous at the lateral side of 

 the transverse section, the region where the rows of hairs will later 

 appear. Figure 22 (Plate 4) shows the anterior and posterior limits of 

 the invagination and the fundament of the sensory ridge, marked by a 

 fold in the hypodermis and chitin at cl. ma. The matrix cells just 

 posterior to this fold, whose processes are directed toward it, are those 

 which are to form the transverse portion of the hair rows. As in the 

 first stage, nuclei of nerve cells lie immediately beneath the matrix cells, 

 but the cytoplasm about them shows as yet no definite boundaries or 

 outlines, nor are there any signs of nerve fibres connected with them. 



c. Third Larval Stage. 



(Chelae relatively larger, uropods present.) 



In this stage (Plates 4, 5, Figs. 21, 23) invagination has proceeded 



