ROTIFERA. 223 



of the alimentary canal occurs late. The stomodaeum (fig. 100) 

 gives rise to the mouth (M), oesophagus and masticatory appara- 

 tus (MS). The mesenteron is formed from the median part of 

 the hypoblast ; the lateral parts of which appear to give rise to 

 the great lateral glandular structures (Id) which open into the 

 stomach, and to the ovaries (?) (ov) etc. The proctodaeum 

 becomes the cloaca and anus (an}. The origin of the mesoblast 

 is not certainly known. The shell is formed before the larva is 

 hatched an occurrence which does not take place till the larva 

 closely resembles the adult. 



The early developmental stages of the male are closely 

 similar to those of the female ; and the chief difference between 

 the two appears to consist in the development of the male being 

 arrested at a certain point. 



The larvae of Lacinularia (Huxley, No. 234) are provided 

 with a praeoral circlet of cilia containing two eye-spots 1 , and a 

 perianal patch of cilia. They closely resemble some telotrochal 

 polychsetous larvae. 



Salensky has compared the larva of Brachionus to that of a 

 cephalophorous Mollusc, more especially to the larva of Calyp- 

 traea on which he has made important observations. The 

 praeoral lobe, with the ciliated band, no doubt admits of a 

 comparison with the velum of the larva of Molluscs ; but it does 

 so equally, as was first pointed out by Huxley, with the ciliated 

 praeoral lobe of the larvae of many Vermes. It further deserves 

 to be noted that the trochal disc of a Rotifer differs from the 

 velum of a Mollusc in that the eyes and ganglia are placed 

 dorsally to it, and not, as in the velum of a Mollusc, within it. 

 The larva of Lacinularia appears to be an exception to this, 

 since two eye-spots are stated to lie within the circlet of cilia. 

 More important in the comparison is the so-called foot (tail), 

 which arises in the embryo as a prominence between the mouth 

 and anus, and in this respect exactly corresponds with the 

 Molluscan foot. 



If Salensky "s comparison is correct, and there is something 

 to be said for it, the foot or tail of Rotifers is not a post-anal 

 portion of the trunk, but a ventral appendage, and the segmen- 



1 In Leydig's figure of the larva, Zeit. f. wiss. Zool. Vol. m. 1851, the eye-spots 

 lie just outside the ciliated ring. 



