THE NEMERTINI 



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Turbellaria ; to these are added, in Cerebr. urticans, nematocysts 

 like those of Coelenterates, except that several are contained in 

 each cell. Below this is a basement membrane, then follows the 

 musculature, which in general repeats that of the body wall, only 

 reversed, being " dimyaric " in the Proto- and Meso-nemertini, and 

 "trimyaric" in the Heteronemertini. Outside, again, comes the 

 flat rhynchocoel epithelium. Behind this tubular, eversible region 

 the circular muscles cease ; but the longitudinal muscles are con- 

 tinued backwards to form a " retractor muscle." In Eupolia this 



FIG. XV. 



Proboscis of Metanemertine. 

 Longitudinal section in the 

 " middle " region in a state of 

 retraction (from Joubin). A, the 

 cavity of the anterior region, 

 capable of eversion ; D, cavity of 

 posterior, retractor region. The 

 cavity of the middle region is un- 

 lettered ; it consists of a dilated 

 "reservoir" communicating by a 

 "duct" with the anterior region. 

 The plane in which the nerve rings 

 lie is the " diaphragm." Ep, epi- 

 thelium of rhynchocoel ; gl, gland 

 cells, whose ducts pass to the 

 "acanthophore," which they prob- 

 ably secrete ; Mce, layer of circu- 

 lar muscles, which are external on 

 eyagination ; Mci, layer of internal 

 circular muscles ; M le, layer of 

 longitudinal muscles, external on 

 evagination ; Mli, internal longi- 

 tudinal muscles ; N, nerve layer in 

 proboscis ; it is continued down to 

 the diaphragm, when it forms two 

 rings round the "duct." P, the 

 epidermal papillae ; S, the median 

 (functional, or chief) stylet, sup- 

 ported on its acanthophore, which 

 is unlettered ; Sa, right and left 

 " acanthocysts " containing lateral, 

 reserve or accessory stylets. 



retractor muscle is hollow, the narrow tubular cavity being con- 

 tinuous with the wider cavity of the proboscis proper ; there are 

 thus in this genus two tubular regions to be distinguished in the 

 proboscis an anterior eversible region, and a posterior non- 

 eversible region. This leads on to the more complicated apparatus 

 of the Metanemertines (Fig. XV.). Here the two regions are quite 

 definite, and are separated by a third, middle region in which the 

 calcareous stylets are developed. The wall of this middle region 

 is much thickened, owing to the development of special muscles 

 and of gland cells ; the canal which puts the anterior and posterior 

 cavities into communication is differentiated into three parts : (1) 



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