442 



T. J, EVANf? 



to Calma. In the account that follows the digestive system 

 claims first place, not only because it shows the most extensive 

 aberration from type but because it provides the key to the 

 meaning of other topographical and histological changes. 



The Digestive System,— The facial aspect of the 

 animal in repose is rather flat and directed forwards and 

 downwards. Below the middle of its smooth surface is a conical 

 depression leading to the small oval mouth. In the act of 

 eating the face fits like a hood on the egg and is capal)le of 

 considerable extension. In this position the animal looks very 

 aggressive, especially when the pressure which results in the 

 swallowing of the eml)ryo is exerted. In the meanwhile the 

 narrow odontophore bearing the saw-like radula has been 

 protruded into the oral opening and the act of slitting 

 the egg-membrane performed. This muscular odontophore 

 (fig. 2, rad.), which is very narrow at the protruded edge, is 

 broadly based J^ehind and on the floor of the buccal cavity. 

 It is covered by a cuticle which is continuous under the radula 

 and with the general liuccal lining. Laterally the buccal 

 cavity is largely occupied by a pair of muscular pads bearing 

 smooth jaws, wdiich are local thickenings of the buccal cuticle 

 (j.). These come into action in the act of swallowing. Between 

 them is a strongly cuticularized ventral groove in which the 

 odontophore moves. The groove continues forward into a 

 cavity in the ventral lip which acts as a reservoir for the very 

 massive buccal glands. 



Previous descriptions of the radula (fig. 3) have represented 

 it as a continuous ribbon finely serrated at the edge like a bent 

 saw, thus contrasting it strongly with other uniseriate radulae 

 consisting of separate teeth carried on a basal dentigerous 

 strip. The examination of transverse sections, however, shows 

 that the profile view obtained in potash preparations is mislead- 

 ing, and that this radula is less of a neomorph than was 

 supposed. It is constructed on the fret-saw principle, teeth 

 not unlike those of the Aeolids being borne on a stout bent 

 cylindrical rod (b.r.) secreted by the bed-cells of the radular sac. 

 The teeth are as usual formed by the roof-cells of the sac and sit 



