BALANOGLOSSUS. 353 



Description of Balanoglossus. 



Form. — The worm-like body consists of a prominent pre-oral region 

 or " proboscis," a firm " collar " behind the mouth, behind this a region 

 with gill-slits, and finally, a long, soft, slightly coiled, ringed portion. 

 It is to the tongue-like shape of the proboscis that the word Balano- 

 glossus refers. 



Skin. — The epidermis is ciliated, and exudes abundant mucus from 

 unicellular glands. In B. robinii the mucus sets firmly. Some species 

 are phosphorescent. 



The Muscular System is best developed about the proboscis and 

 collar, which are used in leisurely locomotion through the soft sand. 

 There are external circular and internal longitudinal muscles. The 

 fibres are unstriped. 



The Nervous System includes a dorsal system, which is most de- 

 finitely developed in the collar, but it extends along the dorsal surface. 

 According to some, the nerve-cord in the collar has a central canal com- 

 parable to that of the spinal cord. But the dorsal nerve-cord in the 

 collar is connected by a band round the pharynx with a ventral nerve. 

 There is also a nervous plexus distributed beneath the skin. The adult 

 has no special sense-organs, nor do we expect them in an animal which 

 spends most of its life immersed in muddy sand. In the larvse of some 

 species there are two eye-spots. 



Alimentary System. — The mouth lies ventrally between the proboscis 

 and the collar, and is adapted for swallowing the sand moved about by 

 the wriggling proboscis and by the collar. The pharynx is constricted 

 into a dorsal and ventral region, of which the former is respiratory and 

 connected with the exterior by many gill-slits, while the latter is nutri- 

 tive, and conveys the food-particles onwards. This ventral region 

 may be compared with the "ventral groove" in Tunicates, with the 

 ' ' hypobranchial " groove in the lancelet, with a similar region in the 

 lamprey, and even with part of the thyroid gland in higher Vertebrates. 

 Behind the region with gill-slits, the gut is digestive and absorptive, and 

 bears throughout the anterior part of its course numerous glandular saccu- 

 lations which can be discerned through the skin. The animal eats its 

 way through the sand, and derives its food from the nutritive particles 

 and small organisms therein contained. 



The Skeletal System, is represented by the " notochord," which lies 

 in the proboscis, and arises like the notochord of indubitable Verte- 

 brates from the dorsal wall of the gut. It is for the most part an 

 anterior outgrowth of the gut. Beneath it there lies a " chitinous " rod, 

 which divides into two in the collar. There is also a delicate " chi- 

 tinous " skeleton about the gill-slits. 



The Body-Cavity is somewhat complex, consisting of five distinct 

 parts, all of which are lined by mesoderm, and arise as pouches from 

 the primitive gut or archenteron. (a) There is first the unpaired cavity 

 of the proboscis, which communicates with the exterior by a dorsal pore 

 (or sometimes by two) at the base of the proboscis next the collar. 

 From this cavity particles may pass out, but none have ever been seen 

 going in. It is possible that a glandular structure, which lies in front 



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