Bd. VI: 4) 
THE ECHINOIDEA. 
75 
SIZ (»Challenger» Echinoidea, p. 177. PI. XX. a.) and recently (under the name of 
Abatus cavernosus) in the »Panamic Deep Sea Echini» (p. 21 1. PI. 99), and also 
LOVÉN has given some excellent figures of a young specimen of the same species 
(named Abatus cavernosus) in his memoir »On Pourtalesia» PI. XIV. The correspond- 
ing stages of A. cavernosus agree very closely herewith, as might be expected on 
account of the near relation between these two species. The following stages, show- 
ing the development of the fasciole and the ambulacra, and, upon the whole, the 
transformation of the test from the embryo to the fullgrown Echinoid were hitherto 
unknown. I have thus thought it desirable to use this rare occasion for giving a 
full account of the changes undergone during growth by this species. — Upon the 
whole the development of A. cavernosus corresponds rather closely with that of 
Brisaster fragilis (comp. »Ingolf» Echinoidea. II. p. ill — 114. PI. XIII), a fact of 
considerable importance for judging of the relationship of the genus Abatus. 
It may first be stated that in one and the same marsupium may be found newly 
laid eggs, embryos with the spines just appearing and fully developed young ones, 
ready to leave the marsupium. 
The first developmental stages: cleavage etc. I am unable to work out satis- 
factorily on the material before me; for this purpose a large material preserved 
specially for such research will be needed. Evidently the early development of this 
species will prove to be very interesting. On account of the large size of the eggs 
(a little over 1 mm. in diameter), there must necessarily be considerable differences 
from what obtains in Echinoids with small eggs and pelagic larvae. As far as can 
be ascertained on the material in hand, the cleavage proceeds in the same way as 
that described by me for Cucumaria glacial/s LjG., the nuclei lying scattered in 
the yolk and later on wandering towards the surface of the egg, the great yolk- 
mass remaining undivided. * 
Regarding the development of the test, the first stage seen is that figured in 
PI. XVIII. Figs. 3 — 4. The embryo at this stage measures 1.5 mm in length. It is 
seen that the development of the actinal skeletal plates precedes that of the abactinal. 
From the actinal side the calcareous plates spread out and, as it were, overgrow the 
yolk. The embryo in this stage is generally more or less irregular in shape on 
account of the mutual pressure in the marsupium; the actinal side, and so much of 
the abactinal side as is already covered by the coronal plates, has assumed the regular 
shape; but the apical skeleton still consists only of small isolated plates, which can- 
not overcome the irregular shape of the embryo. Contemporarily with the development 
of the apical system into a compact covering the embryo acquires its regular shape. 
The explanation of the actinal skeleton (PI. XVIII. Fig. 3) causes no difficulties. 
The primary interambulacral plates are seen to project adorally beyond the ambu- 
* Zur Anatomie und Entwicklung der Cucumaria glacialis. Zeitschr. f. wiss. Zool. LVII, 1894, p. 721. 
