ENTBROPNEUSTA PROM MADRAS. 129 



cavities. Peribranchial cavities are absent. The nerve-cord 

 of the collar does not contain a central cavity, but numerous 

 small spaces are present in it. The anterior end of the 

 collar is sunk into a dee]) pit with fokled walls and lying 

 above the collar-cord. In transverse sections it appears 

 as a crescentic space lying above, and extending laterally 

 over the nerve-cord. The pit extends posteriorly more 

 than half-way into the collar. The collar pores have the 

 usual structure. 



Trunk. — The body-wall is marked, as in other species of 

 the genus, by the absence of a circular muscle. The longi- 

 tudinal fibres form a thick band below the epidermis. 

 Veutrally, on either side of the ventral mesentery, these 

 fibres are scattered in the body-cavity, and a few are closely 

 applied to the gut-wall, forming a feeble longitudinal muscu- 

 lature to it. Radial muscles are well developed between the 

 outer walls of the branchial pouches and the body-wall; they 

 are also scantily developed between the dorsal portion of the 

 gut-wall and the body-wall. In the hypobranchial wall in 

 the posterior part of the branchial region, outside the base- 

 ment membrane of the alimentary epithelium, a thin circular 

 muscular layer, consisting of extremely thin fibres, is visible. 

 These fibres are also present in the epibranchial region. 

 They have not been traced round the branchial pouches. 



In the branchial system thei'e is a histological detail which 

 seems worth noting. A similar condition has been described 

 only in one other Enteropneust, Balanoglossus Kupferi. 

 The ciliated epithelium, instead of forming a single laj^er of 

 low cells, consists of a layer of very thin columnar cells wliich 

 have their numerous minntc nuclei arranged in different 

 levels. In stained preparations the epithelium shows a 

 closely-packed mass of coloured dots, and gives a character- 

 istic appearance to the branchial portion of the alimentary 

 canal. As in other forms, this epithelium is restricted to the 

 anterior and posterior faces of the branchial septa and 

 tongues. The outer wall of the branchial tongue is not 

 folded into the cavit}' of the tongue. 



VOL. 47, PART 1. NEW SERIES, I 



