STROPHOMENIA SPINOSA. 123 



in amount, forming as it does scarcely more than a thin sheath about the 

 innumerable spicules imbedded in it. The latter structures are of two 

 varieties shown (Plate 36, fig. 16), the larger being directed radially. 



The hypodermis, comparatively thin and not especially favorable for study, 

 comprises so far as determined the usual types of cells. Those responsible for 

 the development of the needle-shaped spine retain their attachment with it so 

 long as the spicule remains in the cuticle. The formation of the spines with 

 truncated base has not been followed, but in later stages each rests upon a small 

 knob, probably the remains of a matrix cell, and as noted previously may be 

 withdrawn deep into the somatic musculature. This appears to be a normal 

 process for as noted in a previous paragraph some specimens are smoother ex- 

 ternally than others; but the mechanism by which this is effected is by no means 

 clear since in a few cases only do muscles attach to the base of the spine. The 

 papillae are few in number. 



The anterior pedal gland is a moderately developed organ extending from 

 the level of the brain to the posterior border of its outlet into the pedal furrow. 

 The cells are of the usual pj'riform type and contain a secretion staining inky 

 black with haematoxylin. The outlet is a simple sac-like indentation (Plate 8, 

 fig. 2, and Plate 11, fig. 2), highly ciliated, with the foot springing as a well- 

 developed fold from its dorsal wall. The cells of the posterior pedal gland are 

 unusually numerous anteriorly and save that they are of a somewhat smaller 

 size are not to be distinguished from those of the anterior pedal gland with 

 which they are continuous. Posteriorly the furrow is continuous with the 

 cloaca! chamber. 



The opening of the sensory atrium, subterminal in position, is comparatively 

 wide, and as may be seen (Plate 11, fig. 2 and Plate 8, fig. 2), the atrium itself 

 is imperfectly separated from the succeeding section of the gut. As usual three 

 pairs of sensory ridges are present, the two bounding the cirrose area being well 

 defined while the remaining more exterior one is only faintly outlined. The 

 first two mentioned are comparatively low, not penetrated by blood sinuses, 

 but by connective tissue and muscle fibres and a very few nerve fibres from 

 neighboring ganglia. The cirri are slender, compact bodies united in groups 

 of 3-6. 



The mouth, a relatively wide opening, leads into a tube of great length, but 

 of much the same size and appearance throughout its course. Its epithelial 

 lining is usually fashioned into low longitudinal folds and rests upon a circular 

 muscle layer of moderate thickness external to which are a few longitudinal bands 



