Ecbiuroids from Brackish Water 
331 
may vary from i*002 to 10085 at the begimiing of the dry season. The specimens 
of T. sabinum obtained by the " Skeat Expedition" were dredged from the bottom 
in which, as Dr. Annandale informs me, the mud is mixed with sand, while the speci- 
men procured by Dr. Annandale in 1916 was from mud mixed with dead shells. The 
only specimens of T. dendrorhynchus yet known were collected in the Chilka lyake 
in dense mud from muddy water varying in specific gravity from i'Oo6 to I'oog. The 
type- specimen of T. branchiorhynchus came from the still denser mud and equally 
muddy water of a creek in the Gangetic Delta, the water of which had a specific 
gravity of only 1006. Other specimens of this species have since been taken in 
mud-flats at Balasore on the coast of Orissa. This place may be said to be on the 
open sea, but it must be remembered that the waters of the upper parts of the Bay 
of Bengal are much less salt than those of most seas. 
There seem to be three points, therefore, in which the natural surroundings of 
the three species of Thalassema are abnormal, viz. (i) the low salinity of the water, 
(ii) the fact that it holds a large amount of finely divided mineral matter in suspension, 
and (iii) the density of the mud in which they burrow. Very little precise informa- 
tion is as yet available as to the effect of change of salinity on the respiration of 
aquatic animals, but that it may have a very material effect is probable. In dense 
mud and very muddy water there must inevitably be increased difficulty in obtaining 
the necessary amount of oxygen. 
Integument. — The appearance of the integument has already been referred to in 
the account of the external characters. Its histological structure exactly resembles 
that of the proboscis except for the following differences : — the cuticle is thinner and 
a distinct alveolar layer is present. In the epidermis the gland -cells are far numerous 
in the papillae and very few sensory cells can bs distitigiiish ed. The cutis layer is 
very well developed and fills up the greater part of the basal portion of the papillae. 
The muscular layers are well developed and consist of an outer one of longitudinal, 
a middle one of circular and an innermost of oblique muscle- fibres. The innermost 
layer of the integument consists of peritoneal cells of the type seen in other species. 
Alimentary canal. — No differences can be seen in this group from that of the other 
described species. A short account of it in T. branchiorhynchus is here given and it 
may be noted that but for the differences in the length of the various divisions it is 
similar in the three species. 
The alimentary canal may be divided into three divisions — (i) Fore-gut from the 
mouth to the pre-intestinal constriction and consisting of pharynx, oesophagus, 
gizzard and crop. This portion of the alimentary canal is different from the follow- 
ing second portion in having the longitudinal muscular layer outside the circular and 
not inside it, as is the case in the second part, (ii) Gut proper or the intestine having 
the ciliated groove {C.g) ventrally, this in the middle portion of the intestine becomes 
separate from it and runs along ventral to it as the collateral intestine or siphon {Si). 
This portion is marked off from the last part by the caecum, (iii) Hind gut or rectum, 
which opens at the anus and has the anal vesicles opening into it. 
The mouth opens into the broad pharynx (P) which is 28 mm. long and opens 
