872 
Tetrarhynchus erinaceiis 
the internal wall of the cavity in which the scolex lies—the recepta^ 
culum scolicis. The internal wall of the latter is not cuticularised 
except immediately round the base of the scolex. The latter has all 
the structure characteristic of the mature Cestode and need not be 
described here. Just anterior to the place where the cuticle becomes 
continuous with the wall of the receptaculum the excretory canals come 
near the surface and pass out into the superficial parts of the posterior 
portion of the larva. This posterior part—the so-called “ post-cephalic 
segment ”—corresponds to the bladder of the cysticercoid larva of a 
Taenia but unlike the latter it is not hollow, but is filled with an 
obscure, faintly-staining tissue which resembles the parenchyma found 
in the tissues of the Cestodes; and is apparently reticular in nature, 
with very large interspaces. The excretory canals run backwards, 
loosely convoluted, in the superficial tissues of the “ bladder” and open 
into a minute vesicle which again opens on to the exterior extremity 
of the larva. No other structures are present in this posterior part of 
the larval body. 
Proboscis bulbs 
ExcTotcry 
vesicle 
I 
Proboscides 
ridiurn 
Fig. 2. Longitudinal section of a Plerocercoid larva from the mesentery 
of Arnoglossus laterna. Mag. 50 dia. 
rrovosc/s Dulbs 
Receptaculum scoUcis 
The receptaculum scolicis is really an invaginated part of the outer 
wall of the larva, as one sees in van Beneden’s schematic figures. 
From the bottom of this invagination the scolex forms as a bud, with 
the bothridia and other structures in their definite relations, and not 
with their surfaces reversed as in the case of a Taenia. There should 
be an opening in the wall of the receptaculum through which the 
