156 DONDEKSIA CALIFORNICA. 



fibres, probably miisclo and norvo, judging from other species, pass into the 

 underlying tissue. 



It is evident from the thinness of the cuticle (Plate 32, fig. S) that but a 

 single layer of spicules exists in this species and from spaces in the decalcified 

 cuticle it is evident that they are of small size. The hypodermis is comparatively 

 thick and comprises several classes of cells. The most conspicuous, and at the 

 same time the most rare, are gland cells which are generally more deeply placed 

 than the other elements between which their delicate ductules pass to the exte- 

 rior. These are most abundant on the ventral surface. 



At all points there are almond-shaped spaces in the hypodermis which 

 appear to have been filled with a calcareous product, and judging from the cell 

 remnant usually in connection with them, it is probable that they are spicules 

 in process of formation. The cells apparently responsible for the formation of 

 the cuticle are columnar, non-staining elements containing a centrally placed 

 nucleus. Between them are very slender cells with spindle-shaped mesially 

 placed nuclei which may possess a sensory function though this is not definitely 

 established. No papillae are present. 



In this species the atrial cavity, provided with cirri and ridges, is entirely 

 distinct from the radula-bearing region which communicates with the exterior 

 by an opening immediately in front of the outlet of the anterior pedal gland 

 (Plate 5, fig. 4). In this forward division the dorsal or innermost of the buccal 

 ridges is lacking; the external one on the other hand is prominent and abundantly 

 ciliated. The cirri are united by their bases into groups of three or four or 

 rarely six. In this enlarged basal part it is sometimes possible to distinguish a 

 few bipolar cells which connect with fibres passing distally through the cavity 

 of each cirrus, and in a reverse direction become lost to view in the vicinity of 

 the ganglionic masses surrounding the cirrose division of the digestive tract. 



As just noted this anterior end of the alimentary canal is separated from the 

 succeeding portions by a narrow tract bounded by hypodermal cells and covered 

 with a spiculose cuticle. Whether this division line disappears later in life, the 

 cirrose section then communicating with the remainder of the gut, as is usually 

 the case, it is impossible to say, though judging from the size of the specimen and 

 the profound changes required to bring about such a state of affairs it seems 

 probable that the present arrangement obtains in the sexually mature individual. 

 From the foregoing it develops that what Thiele assumes to be the true mouth 

 is an independent opening communicating in the present instance with a com- 

 paratively narrow plicated tube leading into the larger pharyngeal, radula- 



