66 SUMMARY OF CURRENT RESEARCHES RELATING TO 



ment of the ova takes place at one end only of the ovary, and the 

 deutoplasm is formed by diffusion from the yolk-forming mass to the 

 developing ovum. 



The characteristic sense-organ of Rotifers (absent only in 

 Philodiueje) is found in the lateral sensory bristles at the beginning 

 of the posterior third of the body, which s^vc innervated by nerves 

 running anteriorly on either side to end in the coiled cephalic 

 termination of the water-vascular system. There is no connection 

 between these nerves and the " brain." 



The uniformity of shape in the male Rotifers is important phylo- 

 genetically, as pointing to the primitive type of the class. The 

 nervous, excretory, and rotatory systems are simpler, and the rudimen- 

 tary intestine is not always furnished with calcareous concretions. 

 Copulation does not take place by the vagina ; the penis of the male 

 jierforates the cuticle of the female at any point, and this holds good in 

 cases where six to eight males impregnate the same female at one and 

 the same time. The semen is thus introduced into the body-cavity 

 in which it swims about freely. 



Parthenogenesis occurs, but previous observers were wrong in 

 stating that copulation had any effect in determining the character of 

 eggs laid. Cohn .was mistaken in his view that " winter-eggs " were 

 only laid after copulation. 



" It is also an error to imagine that the Rotatoria can be desiccated 

 by absence of water and recover later in the presence of water. Very 

 many perish in a short time if they come into contact with the air at 

 the surface of the water in which they live." * 



Structure and Development of Myzostoma.f — J. Beard adds 

 considerably to our knowledge of the development of Myzostoma, and 

 throws some light upon the affinities of the group, concerning which 

 there are very different opinions among naturalists. 



The ovary is not a well-defined organ, but more or less fills up all the 

 space left between the skin and the other organs of the body ; the ova 

 when fully mature possess the so-called yolk-nucleus ; before fertiliza- 

 tion two polar bodies are extruded, but in some cases it was observed 

 that the extrusion of these took place after segmentation had actually 

 commenced ; segmentation, as in the Cbastopoda, is complete but 

 unequal, and a gastrula is formed by epibole, six small ej^iblastic cells 

 surrounding a single large hypoblast cell which subsequently divides. 

 The blastopore closes, and at that spot is formed the mouth of the 

 embryo. The larval stage may conveniently be divided into three 

 periods, each of which is considered separately. 



First period. — The newly hatched larva consists of a single layer 

 of epiblast cells surrounding a smaller number of larger cells ; the 

 ciliation is not uniform, but the cilia are arranged in tufts, and are 

 not found in the region of the future anus. The mouth is formed on 

 the second day. 



Second period. — The changes undergone during this period are the 

 development of provisional setcB, and the restriction of the cilia to 



* Cf. snpra, p. b9. 



t MT. Zool. Stat. Neapel, v. (1884) pp. 544-80 (2 pis.). 



