94 BALANOGLOSSIDA. 



branchiogenital region (p. 69). In the simplest cases they form 

 on each side one series, the series of primary gonads, which open 

 to the exterior on the dorsal surface along a line which may be 

 called the gonaducal or sub-median line. The submedian line 

 lies along the insertion of the lateral septum, where the longi- 

 tudinal muscular layer is broken, and in the branchial region 

 frequently coincides with the branchial groove, but in Ptycho- 

 deridae it is placed at a greater or less interval from the branchial 

 groove and the branchial pores perforate the dorsal longitudinal 

 muscles. In the branchial region the primary genital sacs open 

 externally to the branchial pores (Fig. 61), with which they 

 correspond roughly in number. 



The wall of the genital sacs consists internally of a layer of 

 germinal epithelium which is continuous with the ectoderm 

 through the external opening. Outside this is a layer of base- 

 ment membrane containing an abundant capillary plexus or 

 even a continuous sinus. Then come some muscular fibres 

 and a layer of coelomic epithelium towards the body cavity. It 

 would appear that new gonadial sacs are continually being 

 formed at the hind end of the genital region. In Ptychodera 

 asymmetrica gonads are present only on the left side * (Punnett). 



. The arrangement just described, in which the so-called primary series 

 of gonads alone is present, is the simplest found and is possibly charac- 

 teristic of all in the young state. It usually becomes more complicated 

 with growth in the following way. Tho genital sacs become lobed and the 

 lobes acquire independent openings to the exterior (Schizocardium brasil- 

 iense, Glandiceps talaboti). The secondary pores, as these are called, are 

 not placed in the line of the primary pores, but are internal or external to 

 them, except in the branchial region, where there are no secondary pores 

 internal to the primary pores. The secondary pores usually open on the 

 submedian line, but some of them may perforate the dorsal longitudinal 

 muscles. A further complication is reached by the complete separation 

 off of some of the lobes, to form a number of accessory glands in addition to 

 the original gland which is called the primary gland. These accessory 

 glands are not placed in the series of primary glands but in longitudinal 

 rows there may be several either external or internal to the primary 

 row. They acquire their own external openings, which, inasmuch as they 

 belong to the accessory gonads, we shall call accessory pores, to distinguish 

 them from the openings of the lobes of the primary glands, which we have 

 called secondary pores. The accessory pores may be either external or 

 internal or both, and they may be placed on the submedian line or per- 

 forate the longitudinal muscles. 



* Agreeing in this respect with some species of Amphioxus (see Gold- 

 schmidt on Amphioxides, Wiss. Ergebnisse der " Valdivia " Expedition, Bd. 

 12, Lf. 1, 1905, and Zool. Anzeiger, 30, 1906, p. 443). 



