The Morphology of a Solenogastre. 705 
groove. In fully extended specimens (as in Fig. 3) this has no 
connection with the mouth, being merely an inpushing of the body 
wall with cuticle and spicules throughout its entire extent. Where 
the pharynx has been retracted and the muscles attached to the 
blind end of this depression are in a state of contraction the pit 
is deepened to such an extent that the mouth and frontal sense 
organs are drawn within it (Fig. 19). 
As in other members of the family the cuticle is relatively scant 
in amount and the spicules form a single layer. Generally speaking 
these are directed backward making an angle of 30 degrees or even 
less with the underlying hypodermis. Those surrounding the cloacal 
opening are radially arranged, and as Fig. 8 shows, form an angle 
with the spines covering the body generally. In fully developed 
specimens the spicules are triangular or leaf-like in form (Fig. 21) 
and gradually increase in size from those of 0,02 mm length in the 
region of the mouth to others 0,38 mm long surrounding the branchial 
region. 
Internal anatomy. 
The hypodermis is relatively very thin and its cell boundaries 
are frequently invisible. The component cells are cubical in form 
and contain dense centrally placed nuclei. Beneath some of the 
spines a basal cell with comparatively large spherical nucleus is 
plainly visible while in others it has become reduced to a dense 
mass attached to the base of the spicule. Whether other elements 
than the basal cell are operative in the formation of the spicule it 
is impossible to state. 
The musculature of the body wall consists of the usual layers, 
circular, diagonal and longitudinal, which exhibit no peculiarities 
save that they are of less than average thickness. Sections show 
that on each side of the mid ventral line the longitudinal muscles 
are considerably thickened, as they are in Pr. sluiteri (HuBRECHT’s 
fig. 24) or in practically all of the Neomeniidae, and that between 
them a gap exists (Fig. 5), visible as a dark streak in living and 
preserved material, which corresponds to the usual position of the 
foot. In other words the somatic muscles retain their primitive 
condition although all external indications of the foot have disappeared. 
The relatively large mouth opening leads into a canal which in 
fully expanded specimens proceeds almost directly backward to unite 
with the pharynx at the front end of the radula (Fig. 3). Its 
