WITH NOTES ON THE WEST INDIAN SPECIES, 



299 



above and PI. XXXII. Fig. 57). Both Schimkewitsch and Spengel admit the possibility 

 of these pores being related in one way or another to gill-slits. 



There may be two sets of pores, anterior and posterior. The anterior pores, when 

 present, follow close behind the branchial region and the posterior pores occur at the 

 genito-hepatic transitional region. 



Intestinal pores do not occur in the Ptychoderidae. 



What we do find, however, in the Ptychoderidae are the remarkable ciliated 

 grooves of the intestine, with their medially placed covering-pads extending (e.g. in 

 Pt. jiava) from the anterior end of the hepatic region to the posterior end of the 

 abdominal region, but not into the caudal region. In the subgenus Chlamydothorax 

 (as shown by Spengel in Pt. erythraea and as I have found in Pt. flava), the ciliated 

 grooves are not simple longitudinal furrows but undergo metameric or interannular 

 sacculations. These sacculations often approach very closely to the epidermis. They 

 strongly resemble a gill-pouch before its perforation to the exterior such as I have 

 described in Pt. flava. The medial covering-pad often suggests a tongue-bar. (Cf 

 PI. XXIX. Figs. 12—14.) 



It is not unlikely that these sacculations of the ciliated apparatus of the gut 

 in the subgenus Chlamydothorax are homodynamous with the gill-pouch diverticula of 

 the gut and, in this quality, are the vestiges of gill-slits which doubtless formerly 

 extended throughout the greater part or the whole of the trunk. Pari passu with 

 the phenomenon of cephalisation, a process which has always been at work in the 

 evolution of Metazoa, the primarily unlimited gill-clefts became limited to the anterior 

 region of the trunk. 



B. COEXTENSION OF GiLL-SLITS AND GONADS. 



The above conception of the limitation of the gill-clefts to the anterior region 

 of the trunk in correlation with cephalisation and regional differentiation is in accord- 

 ance with what happens in the Craniota. What is not in accordance with craniate 

 traditions is the fact that as a first stage in the process of limitation or localisation 

 of the gonads, they were likewise restricted equally with the gill-slits to the anterior 

 end of the trunk. Whereas in Amphioxus the number of gonads is strictly limited 

 and constant, being laid down once for all during the early adolescent phase of 

 development, in the Enteropneusta the formation of gonads goes on throughout life. 

 As stated by Spengel, the principal point of origin of new gonads of the primary 

 or lateral series is at the posterior end of the gonadial series. As is known new 

 gill-slits arise exclusively at the posterior end of the branchial series. 



It is hardly necessary to dwell at length upon the coextension of gonads and 

 gill-slits since it is such an obvious fact, and is practically implied in Spengel's 

 term branchiogenital region. It is none the less remarkable because it is obvious. 

 The reason why we seldom find exact coextension of gill-slits and gonads is because 

 another factor has been at work which has resulted in the more or less complete 

 emancipation of the gonads from the gill-slits (see below). However, there is one 

 admirable example of complete coincidence of branchial and genital regions, namely 

 Balanoglossus canadensis Spengel. 



