. Elements of Comparative Anatomy. 176 COMPAKATIVE ANATOMY. surface, wlaich may be derived from the paired organs of similar form in the Ann elides. There is either a single pair (Sipunculus), or a few pairs (Thalassema, Sternaspis, Echiurus), in correspon- dence with the slight development of metamerism. Internal open- ings into the body-cavity lie close to the insertion of the tubes into the body-wall^, and in several are of service in the generative function, acting as the ducts for the generative products. The greater part of the tube, that is the caecal portion, which lies behind the inte
. Elements of Comparative Anatomy. 176 COMPAKATIVE ANATOMY. surface, wlaich may be derived from the paired organs of similar form in the Ann elides. There is either a single pair (Sipunculus), or a few pairs (Thalassema, Sternaspis, Echiurus), in correspon- dence with the slight development of metamerism. Internal open- ings into the body-cavity lie close to the insertion of the tubes into the body-wall^, and in several are of service in the generative function, acting as the ducts for the generative products. The greater part of the tube, that is the caecal portion, which lies behind the internal orifice, appears to have an excretory function in the Sipunculidte, and as a rule, is distinguished by its brown colour. In others the whole tube serves merely as a duct for the generative system. In most Gephyrea these organs have each and all similar functions, but in some cases there is a division of labour (Sternaspis), the posterior pair of tubes being in connection with the generative, and the anterior with the excretory, function; so that the difference of function which appears in different genera, is here expressed in the same individual. § 145. So far as the structure of the excretory organs is concerned, few fresh characters appear in the Annulata. The organs correspond to the metamerism of the body, for they are regularly distributed on either side of almost every one of its segments. They have therefore, though with but little reason, been called "segmental organs;" a name which is just as suitable for many other organs. Each of them consists of a closely-coiled or loop-like canal (looped canals), which has an internal opening, often peculiar in form and always ciliated, and which at the other end opens on to the surface of the body. This canal is sometimes similar in character throughout its whole length, or but slightly differentiated; frequently several segments may be made out in it, which generally correspond with those already described in the
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