. Comparative anatomy and physiology. 286 COMPARATIVE ANATOMY AND PHYSIOLOGY. into it. In the Annulata it often becomes of con- siderable thickness and is then traversed by pore-canals. In the sedentary marine Annelids a tube is developed as a means of protection \ the inner portion, which is partly membran- ous and partly fibrillated, is formed by special glands in the body wall; out- side this the tube is often rendered moreresistent by the deposition of calcareous matter (as in Serpula), or of aggregations of sand, mud, and other foreign ma- terial (as in Sa- bella, or Amphi- trite), which
. Comparative anatomy and physiology. 286 COMPARATIVE ANATOMY AND PHYSIOLOGY. into it. In the Annulata it often becomes of con- siderable thickness and is then traversed by pore-canals. In the sedentary marine Annelids a tube is developed as a means of protection \ the inner portion, which is partly membran- ous and partly fibrillated, is formed by special glands in the body wall; out- side this the tube is often rendered moreresistent by the deposition of calcareous matter (as in Serpula), or of aggregations of sand, mud, and other foreign ma- terial (as in Sa- bella, or Amphi- trite), which are taken up by the tentacles of the worm, and laid down on the tube by the animal it- self. Within this tube the inhabitant may be retracted, and some (as Sabella) form an operculum by means of which the entrance to it may be closed. In the Sabellidse special cartilaginous supports are developed within the gill tentacles; this is not found in the Serpulidse. The cells of the integument often give rise to hard projecting structures, which may have the form of. Fig. 119.—Gelatinous Tissue from the Disc of Aurelia aurita; a, fibres; b, cells. (After M. Schultze.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bell, F. J. (Francis Jeffrey), 1855-1924. London, Cassell
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