88 Ech. 
ECHTNODERMATA. 
wards the four longitudinal muscles of the lateral radii, which follow in 
the order of their appearance the relations of the radial vessels and nerves. 
The splitting off of the retractor muscles from the longitudinal ones 
takes place very late. The calcareous bodies are visible in the larva, and 
there is no special larval skeleton. Each calcareous body originates as a 
rod, the ends of which bifurcate repeatedly at angles of 120°, forming a 
lattice plate. The meshes of the plate each contain several cells. The five 
anterior plates are arranged radially, and form a sheath for the tentacles, 
which correspond to the line of contact between two plates. These five 
plates are in contact posteriorly with five others ; but soon the plates 
increase in size and number, and become imbricate. A second kind of 
calcareous plate appears about the hundredth day. The calcareous ring 
is formed from the body wall, and its radial ossicles show relations to the 
ambulacra] ossicles of Starfish. 
The ectoderm and mesenchyma in young Cucumarians form a single 
tissue, which does not differentiate till later into a distinct epithelium 
and subjacent connective tissue. 
The blood- vascular system is traceable to remnants of the segmenta- 
tion cavity, appearing first as a space between the visceral layer of the 
enterocoele and the endoderm of the mid-gut. In the interradii, a gap 
remains between the enterocoele and the body-wall, forming the large 
lacuna described by Herouard. The folding of the intestine begins to 
be marked on the ninth day, and follows the same course as in the adult 
animal. The stomach is constricted off from the mid-gut. 
Ludwig (4) describes the early development of Cucumaria planet, and 
contrasts his results with those of Selenka, with which they are very 
much at variance. The egg contains a first segmentation nucleus, but 
nothing was to be observed of S.’s “ Kernkeimen.” In the blastula, cells 
wander in to form the mesenchyme, not only at the spot which 
becomes the fundus of the future archenteron, but at any other spot, 
either from ectoderm or endoderm. The gastrula is complete at the end 
of the second day. Mesenchymatous dermal and intestinal muscular 
layers do not exist. The archenteron is bent towards the ventral side, 
and the blastopore is not terminal, but slightly ventral. At the end of 
the third day thohydro-entcrocoelG is separated from the gut, and divided 
into a hydrocoele and two enterocoeles. The hydrocoelo forms an 
irregular horse-shoe with some slight bulgings. The cephalic lobe is 
now formed, and behind it is a slight depression, which becomes the 
oral atrium, and furnishes the epithelium of the tentacles and the 
rudiments of the ring and radial nerves. The oral pit is at first sur- 
rounded by ciliated ridges, representing a nearly suppressed Auricularia 
stage. At the end of the fourth day the hydrocoele has become a ring 
and bears the stone canal, and rudiments of the five radial canals 
giving off the five primary tentacle canals. The point of closure of the 
ring is probably on the right side of the body. The Polian vesicle lies 
on the left. The rudiments of the two first feet appear simultaneously, 
but the water-vascular system does not at first take part in their formation. 
