370 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



vided with two boldly marked eyes, and other differentiated tissues, revolves 

 rapidly within its capsule. This evolution of the ova in these decaying adults is 

 a feature analogous to the elaboration of the respective generative products in 

 the headless fragments of male and female specimens of Linens longissimus and 

 others— the last efforts of the parental tissues being devoted to the reproduction 

 of the species. 



In Tetrastemma variegatum the ova are found in the body of the adult in 

 June and August, and are deposited freely in the vessel. The same changes 

 ensue in the egg as in the other forms, and the young are found in swarms beside 

 the adults in the beginning of July and September. These young forms (Plate 

 IX. fig. 15) are so mobile, that one scarce sees the body of the same shape for 

 two consecutive seconds. The surface is coated with long cilia, by whose aid 

 they are piloted through the water like infusorial animalcules ; while, in addi- 

 tion, they are furnished with a single long tuft anteriorly, as described by M. 

 Van Beneden, in the young of his Polia involuta. The cutaneous textures are 

 not distinguishable as separate layers, and the entire body has a cellular appear- 

 ance, probably from the individual elements of the digestive cavity and the cuti- 

 cular areolae. No eyes are visible. About a week afterwards considerable 

 progress had been made in size, but the cilia had become shorter in proportion to 

 the bulk of the animal ; and though the anterior and posterior ends showed a 

 few conspicuous cilia, the long tuft was absent. There are now four eyes. In 

 another week the stylet-region of the proboscis is nearly complete, the lateral 

 often appearing before the central stylets. The usual mode by which the proboscis 

 escapes under pressure is by rupture per anum. Thus there is a slight divergence 

 in the development of this species, whose young move freely as eyeless organisms, 

 each provided with a long ciliary tuft; while in 0. alba two well-marked eyes 

 appear in the young in ovo. 



Dr Schultze* first observed that his Tetrastemma obscurum was viviparous. 

 He likewise stated that, in the development of the proboscis, the lateral stylets 

 appear before the central, and as the animal grows older, he figures it with two 

 loose stylets lying in the pit of the proboscis — an arrangement, as he supposes, 

 for the supply of the central apparatus. I have also seen a loose stylet or two 

 lying in the anterior chamber of the proboscis, but this occurred both when there 

 was, and when there was not, a stylet on the central apparatus. The physiology 

 of the region, as previously explained, demonstrates that there is no connection 

 between the lateral and central stylets, save perhaps in the composition of the fluid 

 with which both are bathed. Prof. Kefersteinj again details the development of 

 Prosorhochmus Claparedii — a species in which theyounganimals attain considerable 

 advancement before extrusion, for they are found with four eyes, a well-developed 



* Op. cit. p. 65, tab. v. figs. 7, 8, and 9. t Op. cit. pp. 89 and 90, taf. vi. figs. 2 and 3. 



