a Parasite o/Cydippe densa. 31 



The cirri of the feet and the spots become more conspicuous 

 in the next or fourth stage, in which the antennee are better 

 marked, the eyes enlarged, the number of segments increased 

 to nineteen, and the body attains the length of 4 millims. 



But it is in the fifth stage that the structure of the eyes is 

 best seen, as they now appear surrounded by several layers of 

 cells, the nuclei of which are easily rendered visible by means 

 of an ammoniacal solution of carmine, and which are probably 

 of nervous matter, composing as they do that layer external 

 to the choroid which exists, as is known, in the adult Alciope 

 as well as in many Mollusca, the Cephalopoda and Heteropoda 

 for instance, in which the gangliary portion of the retina is 

 seen posterior to the choroid. It is in this stage (distinguished 

 further by the appearance of the capillary setas) that we were 

 enabled to distinguish the dorsal vessel with the perfectly 

 transparent blood. 



In the sixth stage, the four antennge are still more produced, 

 and the choroid appears completely lined with pigment, and 

 composed of grains disposed in perfectly regular series. Be- 

 sides the nervous layer composed of cells of which we have 

 spoken, another layer is visible, exterior to this, surrounding 

 the entire bulb, which, though composed of cells resembling 

 those of the nervous matter, is analogically to be considered 

 a sclerotic. The crystalline is evidently enlarged, and beyond 

 the nucleus presents the appearance of stratification. The 

 larvse in this stage measure 5 millims. in length, and have 

 from twenty to thirty segments. The feet of the first three 

 pairs, which evidently correspond to the original feet of the 

 larva in its first stage, appear smaller than the rest, and con- 

 sist of a stump, deprived of the setje and sheathing fine acicular 

 darts, and of two cim, the dorsal one conical, the ventral short 

 and broader in proportion. The other feet have become more 

 developed ; they are conical, with a dorsal cirrus in the form 

 of a pedunculated oval plate, and a smaller ventral cirrus, be- 

 sides a dorsal tubercle Avith scattered pigment-cells, the rami- 

 fications of which are interlaced in an intricate manner. The 

 setaj are of two sorts, — the first numerous, capillary, simple, 

 flexible ; the others larger, one of them projecting a little from 

 the foot, with a surface armed with very delicate spinules, 

 while the other, of similar structure, remains concealed in tlie 

 interior of the foot, like a dart with the point only a little ex- 

 serted*. 



* Setae and a surface beset with very minute spines have been described 

 by one of the authors in a larva of a Dorsibranch, as yet undetermined, 

 which has some points of analogy to the one in question (Claparede, 

 Beobacht. t. vi. p. 77). 



