ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
439 
the first pair communicated by water-pores with the exterior ; the central 
nervous system consisted of an elongated nerve-ring which lay exactly 
underneath the circumoral ciliated ring. 
The Echinoderma were derived from this hypothetical ancestor by 
a series of changes mainly correlated with the secondary assumption of a 
radial symmetry ; the right anterior enterocoele atrophied, and the nerve- 
ring became separated from the circumoral ciliated band. 
In Chordata the circumoral ciliated ridges of the two sides approxi- 
mated dorsally and fused along their entire length in the mid-dorsal 
line, and constituted a canal which was ciliated internally, and com- 
municated with the gut by means of the blastopore (neurenteric canal). 
The prceoral pigmented pits represent the optic vesicles of Vertebrate 
embryos. The number of enterocoeles became greatly increased with the 
evolution of the Chordate type. 
The Enteropneusta are to be derived from the hypothetical Auri- 
cularian ancestor by a rather more complicated series of changes ; the 
ciliated bands fused only in the middle region of the body, which 
eventually became the collar region of the adult ; this limitation of the 
area of fusion explains the curious fact that the prae-oral sense-organs 
are not enclosed in a medullary tube as they are in Vertebrates, but 
remain on the external surface of the body, and also that there is no 
neurenteric canal in Balanoglossus, where the blastopore persists as the 
anus. It is probable that an adoral ciliated band, comparable to that 
of Auricularia, persists in some form of Tornaria ; Mr. Ritter’s recent 
discovery of a ventral ciliated (?) tract suggests to Mr. Garstang that 
Mr. Ritter has very probably come across the desired homologue of this 
adoral band. 
B. INVERTEBRATA. 
Deep-sea Invertebrates of Indian Ocean.* — Dr. A. Alcock reports 
on a small but interesting collection ; Cerianthus and Cyathohelia are 
for the first time reported from the Bay of Bengal. Dipsacaster (D. 
pentagonalis sp. n.) is a new genus of Astropectinidse, Astroschema 
flosculus is a new species from the lately discovered coral bank north of 
Madras ; Echinolampas , though known as a Sind fossil, has not till now 
been found living in the Bay of Bengal ; interesting Mollusca and Crus- 
tacea were also found. 
Dimorphism due to Parasitism.f — Prof. A. Giard points out that 
the variations of crabs and the like, which require double “ curves of 
Gallon ” to express them, may be, and in some cases seem demonstrably 
due to the dimorphism which the presence of parasites produces. Bio- 
logical statistics alone, as elaborated by Bateson and Weldon, are 
insufficient to solve the problem ; a minute examination of each case is 
necessary. Besides parasitism, other ethological factors may determine 
multiple states of biological equilibrium on which segregation and 
selection operate. In the case of dimorphism due to parasitism, the 
condition of the infested forms is generally, to use his new term, 
paedomorphic. 
* Journ. Asiatic Soc. Bengal, lxii. (1894) pp. 169-77 (1 pi.), 
t Comptes Rendus, cxviii. (1894) pp. 870-3. 
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