24 
Transactions of the Society. 
being ordinarily four to eight, and of various lengths, some being very 
long and even extending into the legs as in Pycnogonida, but not to 
the extent found in those very small-bodied creatures. In the great 
predatory group which comprises the Trombidiidae, Hydrachnida?, 
Bdellinae, Cheyletinae, &c., the caeca are as a rule short and shallow, 
and ill-defined from the ventriculus itself, which covers the greater 
part of the dorsal surface of the abdomen ; but in my investigation of 
Thyas petrojohilus, one of the Hydrachnidae, last year, I was surprised 
to find it possessed of a ventriculus, which, as far as I know, is quite 
unique among the Acarina hitherto investigated. It may be described 
as a hollow square, the outer edges of the square being lobed and the 
inner straight ; if it were round and we could call it a flat ring its 
form would perhaps be more readily understood ; but it is not round, 
and hollow square is the only expression. Dorso-ventral muscles and 
some of the abdominal organs pass through the hollow of the square. 
The lumen is quite continuous throughout. This is a very remark- 
able ventriculus ; yet if we consider I think we can imagine how it 
may have arisen ; suppose we start with a small ventriculus such as 
that of Gamasus, and instead of its rather numerous caeca give it the 
two large caeca of Nothrus theleproctus ; these turn inward a little 
posteriorly ; if they were prolonged they would touch each other at 
their ends, which might well coalesce ; we have then only to suppose 
the coalesced part to be pierced through, so as to make a continuous 
lumen, and we have the ventriculus of Thyas petrophilus ; it is true 
that we must suppose the coalescence to be so complete as not to leave 
a trace of the join. 
The hindgut is the last division of the canal, and is probably seen 
in its typical conditions in the Oribatidae ; the ventriculus is followed 
by a short, ill-defined tract which may be considered to be a small 
intestine, then we have a well-marked and perfectly distinct colon and 
rectum. In other families the small intestine is mostly lost. In the 
Gamasidae and Tyroglyphidae two so-called malpighian vessels enter 
the canal just in the narrow constriction between colon and rectum ; 
in the latter family they are short and small, in the former long and 
important, often running forward to the very front of the body, where 
they end blindly. A serious question arises whether these malpighian 
vessels of the Acarina, and some other Arachnida, can be considered 
to be either the homologues or analogues of the tubes which bear the 
same name in Insects ; in that order these organs spring from the 
canal immediately behind the pyloric end of the ventriculus, that is to 
say, at the very commencement of the hindgut ; in the Acari they 
spring from behind the termination of the colon, immediately adjoin- 
ing the rectum, even if they be not processes of the rectum itself ; thus 
the homology is certainly doubtful. As to the analogy which has 
probably governed the name, at one time it was considered that the 
Malpighian vessels of Insects had a hepatic function ; for a considerable 
number of years, however, this idea has been abandoned, and com- 
