THE BDWABDSIA-STAGE OF LEBRTTNIA. 285 



pairs. Gronidial grooves are absent. An exceptionally well- 

 developed nervous layer occurs, without any accompaniment o£ 

 muscular fibres, and granular gland-cells are numerous. 



3. Larval Coelomic Spaces and Formation of the Qastro- 

 ccelomic Cavity. 



Transverse sections tbrougb the upper archenteric region of 

 the youngest larvae, before any disorganization of the internal 

 tissue has commenced, reveal very narrow, isolated, slit-like spaces 

 within the intermesenterial, parenchymatous tissue, one near 

 the middle of each face of a mesentery (PL 18. fig. 12, d, e,f). A 

 little lower the two within each chamber become connected by a 

 transverse channel, and still lower only a single median cavity 

 occurs in each radial division (fig. 12, y, k, c), the lateral boun- 

 daries always remaining close to the face of the two enclosing 

 mesenteries. 



Approaching the inner termination of the archenteron, where 

 three pairs of the mesenteries have become free, each cavity 

 broadens somewhat, and communication is established between 

 the five chambers on the dorsal or sulcular side of the sulco- 

 lateral mesenteries, and also between the three on the ventral or 

 salcar side, two distinct sinuses being thus formed (fig. 13). The 

 successive steps in the sinus formation, following the disappear- 

 ance of the centripetal parts of the mesenteries, are shown in 

 fig. 12, in connection with the intermesenterial spaces b, a, h. 

 In this particular instance the sulcar mesenteries are the first 

 to lose their connection with the archenteron and permit of 

 communication. 



Fig. 13 represents the arrangement of the various cavities at 

 the inner end of the archenteron. It delineates the details 

 observed in several sections of a polyp which happened to be 

 cut slightly obliquely to the vertical axis. The archenteric tube 

 terminates in four canals, one of which communicates with the 

 sulcar sinus and three with the sulcular sinus. Were the sulco- 

 lateral pair of mesenteries developed only to the same extent as 

 the others, it is obvious that a complete circumferential sinus 

 would be established. 



For a short distance below, the two sinuses remain distinct,. 

 but later become sub-divided into eight V-shaped chambers, 

 associated with the free edges of the mesenteries. 



LINN. JOUEN. — ZOOLOGY, VOL. XXVII. 22 



