PEOEESSOE WYYILLE THOMSON ON THE ECHINODEEMATA. 399 
double ciliated fringe is disposed essentially as in the previously de¬ 
scribed species. 
10. In many of the specimens observed by Muller, the young 
starfish was already developed, attached to the pseudembryo, on the 
dorsal surface of the posterior extremity, above the arms. 
The dorsal surface of the disk of the starfish was turned towards 
the body of the pseudembryo, to which it was attached by one of 
its interradial spaces. The mouth of the starfish, which was still 
closed by a continuous membrane, and the nascent ambulacra and 
water-feet, were turned from the pseudembryo, so that the axis of 
the starfish was oblique to that of the latter.* The oesophagus 
of the pseudembryo entered the 
body of the starfish on the dorsal 
surface; near the centre of one of the 
lower interradial spaces it pierced 
directly into the stomach, a round 
well defined sac, which had pre¬ 
viously been the special stomach of 
the pseudembryo, but which was at 
this stage the common digestive 
cavity of the starfish and the pseud¬ 
embryo. The stomach of the star¬ 
fish then passed by a rapid constric¬ 
tion into the narrow intestine, which 
ended in the interradial space to the 
left of that occupied by insertion of 
the oesophagus, and in the position 
finally assumed by the anal pore of 
the mature starfish. 
At this period then, the whole assimilative system was common 
to the pseudembryo and the Echinoderm embrjo; but the stomach 
and intestine were the permanent organs of the Echinoderm, while 
the mouth and aesophagus were special to the pseudembryo. 
11. Muller observed two other openings in the perisom of the 
starfish, corresponding to points of communication with the body of 
the pseudembryo—one immediately above the orifice of attachment 
of the oesophagus, near the centre of the disk, the other immediately 
beneath that orifice, and closer to the margin. All these three 
orifices were in the same interradial space. In the Northern speci¬ 
mens Prof. Muller could not satisfactorily make out the relations 
*Fig. 1 . Bipinnaria. asterigera (Sars). Ventral aspect of the enlarged posterior 
extremity of the pseudembryo and dorsal aspect of the echinoderm embryo, with 
their special and common organs of nutrition. 
a. Posterior ventral fold of the dorsal integument of the pseudembryo. 
b. Posterior margin of the ventral shield. 
c. Pseudostome, in the transverse groove between the dorsal and ventral fringes. 
d. Oesophagus of the pseudembryo. 
e. Stomach, common at this stage to the pseudembryo and starfish .—(After 
Muller.) 
