160 ICHTHYOMENIA POROSA. 



thickened edges especially on the rounded extremity. Scattered fairly regu- 

 larly among the first variety are those of the second type, paddle shaped, with 

 short handle and a length of 0.054 mm. Along the ventral furrow they are of a 

 greater length, 0.01 mm. 



The hypodermis is apparently one cell thick but the species is peculiar in 

 having the layer developed into many transverse folds (Plate 32, fig. 7) especially 

 on the dorsal surface, and in section these ridge-like elevations render the cell 

 relations obscure. These wrinkles are more pronounced in some specimens 

 than ill others, and are usuallj' more prominent in the anterior half of the body. 

 In some cases thej" are doubtless due to reagents l)ut usually they are certainly 

 normal. The ordinary type of hypodermal cell is very slender, especially in the 

 ridges and is provided with a relatively dense subcentrally placed nucleus. 

 Accompanj-ing these are numerous larger, more globular cells, apparently in 

 large measure empty in preserved material. This may be due to the precipita- 

 tion of some highly watery secretion, or more probably to the decalcification of 

 some calcareous product. 



This species possesses upwards of fifty remarkable organs, apparently 

 sensory, located chiefly about the anterior end of the body in front of the outlet 

 of the anterior pedal gland. All are situated in the ventral half of the animal. 

 Each consists of an invagination of the hypodermis (Plate 24, fig. 12) with an 

 average depth of 0.1 mm. The lining ceils are low, very indistinct and are 

 provided with what appear to be very long cilia, which in most cases extend 

 slightly beyond the general body surface. In the most favorable specimens 

 delicate fibres attach to some of the cells but on the other hand they have never 

 been traced to any undoubted nerve. It is impossible to determine their func- 

 tion j^et it maj' be that in life thej^ act as tactile organs like the apical tuft in 

 the trochophore larva. 



The anterior pedal gland is comparatively large (Plate 24, fig. 1) and occu- 

 pies much of the visceral cavity between the atrium and the forward boundarj'^ 

 of the stomach4ntestine. Its cells are arranged in large groups and are filled 

 with a uniformly granular, lightly staining secretion that after its escape appears 

 as a viscous, darkly staining substance. The posterior p<>dal gland consists 

 of cells filled with a darkly staining, finely granular secretion clearly distinguished 

 from that of the foregoing group, .\nteriorly it rests against, and opens through, 

 the posterior wall of the outlet of the anterior pedal gland, and more posteriorlj' 

 forms a thin sheet resting against the ventral body wall and opening between 

 five folds in the ventral furrow. Posteriorlj' these folds very soon disappear 

 save one, the foot, and the accompanj-ing glands diminish greatly. 



