ON THE DEVELOPMENT OF SAGITTA. 367 



an embryonic character, but now they develop into muscle- 

 fibres arranged as in the adult, with special reference to the 

 movements of the mouth and the action of the hooks. 



Behind the head striking changes are also taking place. 

 The abdominal ganglion is becoming somewhat reduced in 

 size, and at each end of it the ectodermal cells assume a 

 remarkable structure, which is especially pronounced in the 

 neck region, but occurs also at the posterior end of the 

 ganglion, between it and the fin, Avhich now extends con- 

 siderably further forward than the genital cells. The ecto- 

 dermal cells in the parts indicated lose their cell-contents 

 almost entirely, and become so vacuolated as to resemble the 

 parenchyma of a plant in appearance; this occurs most 

 markedly at the sides of the body, but to a less extent 

 dorsally and veutrally also (PI. 20, figs. 18, 19, pt.). The 

 cells so modified resemble exactly the curious vesicular tissue 

 found in the trunk of Spadella draco, the only difference 

 being that the cells in the Sagitta larvee are much smaller 

 and are found in a much less area. This parenchymatous 

 tissue is most prominent about the fourth and fifth days, but 

 it persists as long as I have been able to keep the young 

 animals alive (fifteen days). 



In the tail region the fin has become divided into two 

 parts, a tail-fin (/. t.), extending a considerable distance 

 forward along the body, but separated by a gap from the 

 lateral fin of each side. In the gap is the large tactile organ 

 mentioned above, situated on a prominent ectodermal swell- 

 ing (fig. 18, t. 0.). There is at this stage no trace whatever 

 of the anterior pair of fins of the adult, so that the young 

 animal closely resembles the genus Krohnia, in which they 

 are permanently absent, and in which the tail-fin extends 

 forward as in the young Sagitta. 



The most important changes which take place during the 

 third and fourth days are those affecting the ccelom. The 

 solid condition of the mesoderm described above persists 

 until the third day after hatching, but on that day, unless 

 development is retarded by cohl weather, the mesoderm cells 



