30 Ech. 
ECH1NODERMATA. 
intercalary plates of new origin. The five interradial plates of (3) are 
the basals. The radial pieces of (3) are, in order of appearance, the 
second or “ basilar radials,” while the terminal of the arms are the primi- 
tive or “ terminal radials.” The skeleton of the arms is formed between 
these two radial plates. It is doubtful if the basilar radials are the 
homologues of radials in Crinoich. 
The author further describes the organization of young foetuses of 
Asterias spirabilis. The mouth does not open till a late period. The 
walls of the digestive sac are full of vitelline globules. The relations of 
the tubular organ, plastidogenou3 body (ovoid gland), and hydrophore 
tube (stone-canal) are carefully studied by sections. The plastidogenous 
body appears to be at its origin a nearly cylindrical diverticulum of the 
peritoneal membrane of the intestine, which descends along the hydro- 
phore canal. It is shown that the plastidogenous body has, however, a 
double origin. It is formed in part by a membrane limiting a schizo- 
ccelic cavity, a (pi. v), which is therefore of mesodermie origin, and in 
part from the peritoneal membrane, which is of endodermic origin. It 
is in continuity dorsally (aborally) with the peritoneal membrane of the 
digestive tract, and ventrally with the septa of subambulacral cavities, in 
which are hollowed out in many species the blood-vessels of Ludwig, for 
which the author proposes the name “ plastidogenous tubes,” or “ tubes of 
Ludwig.” 
It is shown that the madreporic sieve of Asterias spirabilis is the result 
of the transformation of the primitive dorsal funnel, which increases 
irregularly, so that its walls are folded, and the folds fuse to form 
sinuous tubes ; these tubes converge towards the summit of the hydro- 
phore canal, and some open directly into it, others into the tubular organ. 
At the junction of the hydrophore canal and the madreporic sieve, the 
former has a lateral opening, placing its cavity into communication with 
that of the tubular organ. The plastidogenous body, or at least the 
membrane of which it is a differentiation, takes part in the formation of 
the calciferous tissue surrounding the hydrophore canal, and of the 
dorsal skeleton in the region adjoining the madreporic sieve. The 
plastidogenous body also takes a part in forming corpuscles for the body 
cavity. 
The arrangement of the subambulacral cavities and labial ring is 
described. The nervous system consists, in a young specimen with rays 
3 mm. in length, of an external cellular layer, a, a middle layer of 
longitudinal fibrils, 0, and an inner cellular layer, y. a is entirely 
epithelial, and composed of rounded and fusiform cells. Some of the 
cells only are prolonged into vertical fibres traversing 0. The cells of y 
are multipolar, and prolonged into fine filaments, of which some plunge 
at once into 0, and others are continuous with the filaments of a fine 
network, which unite amongst themselves the fibrous prolongations of a. 
The multipolar cells of y are ganglion cells ; the cells with fibrous pro- 
longations of a are sensory, and the fibres of 0 are probably nervous, but 
may possibly be connective tissue. 
