. Outlines of zoology. Zoology. 172 UNSEGMENTED "WORMS: with a superficial nerve plexus. Tactile papillae and patches are often present; eyes and eyespots are general; arid in some there are otocyst-sacs. Apart from the cephalic slits, the head also bears sensory grooves and terminal sensory spots. The mouth is ventral, and leads into a plaited glandular fore-gut or oesophagus, which is followed by a straight, ciliated mid-gut or intestine, with regularly arranged lateral caeca. Between the caeca run transverse muscle partitions. The anus is in most cases terminal. In a cavity along the d
. Outlines of zoology. Zoology. 172 UNSEGMENTED "WORMS: with a superficial nerve plexus. Tactile papillae and patches are often present; eyes and eyespots are general; arid in some there are otocyst-sacs. Apart from the cephalic slits, the head also bears sensory grooves and terminal sensory spots. The mouth is ventral, and leads into a plaited glandular fore-gut or oesophagus, which is followed by a straight, ciliated mid-gut or intestine, with regularly arranged lateral caeca. Between the caeca run transverse muscle partitions. The anus is in most cases terminal. In a cavity along the dorsal median line there lies the remark- able proboscis. It is protruded and retracted through an opening above, or, in a few cases, within the mouth. It arises as an invagination from in front, and is a muscular, very richly innervated tube, some- times protruded with such force that it sepa- rates from the body, and then often retains its vitality for a long time, as if it were itself a worm. It has been compared in its retracted state to a glove-finger drawn in by two threads attached to its tip, the threads being retractor muscles. But in front of the attachment of the retractor muscles there is a non- eversible glandular region which secretes an irritant fluid. In many cases there are stilets at the tip of the eversible portion, and if these be absent, there are stinging cells or adhesive papillae. There is a hint of a similar structure in some Turbellarians, and the organ may be interpreted as one which was originally tactile, but which has become secondarily aggressive. It is protruded by the muscular contraction of the walls of the proboscis sheath, which forms a closed cavity surrounding the proboscis, and con- taining a fluid with corpuscles (Fig. 75). In the majority there are three longitudinal blood vessels. Fig. 77.—Transverse section of a simple Nemertean [Carinella).—After Burger. , Dorsal nerve ; proboscis cavity; g., gut; cm., circular muscles;
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
