PROFESSOR WYVILLE THOMSON ON THE ECHINODERMATA. 411 
cimens described were only from 1/lOth to l/5th of a line in diame¬ 
ter. They were discoid, the upper surface arched, the lower flat, 
and from the centre of the lower surface a large open oesophagus 
depended. These zooids might have been taken from their form for 
young pulmograde Medusae, only they moved by the action of cilia. 
Several coecal appendages, ciliated externally, and varying in number 
in different individuals were attached to the lower surface of the disk, 
and round the margin were distributed from two to four extremely 
minute open tubes. 
The stomach, ciliated within, was placed in the centre of the disk. 
Purther advanced the disk became octagonal and crenated. No 
calcareous particles were detected. Muller suggests, that this may 
have been an early stage in the development of Aster acanthion tenni- 
spinosus , a species with from six to eight arms, and three or four 
madreporic tubercles. 
IY.—Pteraster militaris and Asteracanthion glacialis. 
35. Another, and a very distinct form of development was ob¬ 
served by Messrs. Koren and Danielssen, in a well-known Northern 
species.'* 
In Vteraster militaris (M. & T.) a delicate membrane, sup¬ 
ported by the spines above the surface of the perisom, and perforated 
near the centre for the anal aperture, forms a tent-like marsupium 
over the whole of the dorsal surface of the disk. In this cavity from 
ten to twenty eggs are hatched on each individual, and the embryos 
remain until they have assumed their definite radiate form. The 
authors were unable to trace the earliest stages in the development 
of the pseudembryo. In the youngest individuals observed it had 
attained its perfect form, and some indications were visible of the 
special organs of the young starfish.f The pseudembryo (Pig. 4, A.) 
was oval and somewhat flat¬ 
tened, with a slightly undulated 
margin. At the anterior extre¬ 
mity, a large round buccal aper¬ 
ture led into a straight narrow 
intestine, which traversed the 
long axis of the body somewhat 
towards one side, and appeared 
to be lost near the posterior ex¬ 
tremity in the granular substance. Near the centre of the body, a 
rounded mass of darker granules was encircled by a delicate vascular 
* “ Fauna littoralis Norvegiae,” Andet Hefte.—Bergen, 1856. 
f Fig. 4. Pteraster militaris (M. & T.) A. Pseudembryo, a. pseudostome. 1). 
intestine, c. vascular coecum indicating the sand-canal and madreporic tubercle, d. 
central vascular ring of the ambulacral system. B. A farther stage showing the 
outline of the Echinoderm embryo, a. pseudostome. b. pseudocele. b. pseudoproct, 
d. central ring of the ambulacral system of the star-fish. c. nascent sand-canal and 
madreporic tubercle. (After Koren and Danielssen.) 
