Haddon — The Neivly- Hatched Larva of EuphyUia. 129 



This terminology may be complicated, but it is definite, and I 

 only employ two new terms, and these merely replace others. 



Transverse sections show that the only difference in the 

 mesenteries between our larvse and the corresponding stage in 

 many Actiniae is that the sulcular directives, although they reach 

 the oesophagus, are devoid of mesenterial filaments (craspeda), 

 being in this respect in the same condition as the fifth and sixth 

 pairs of mesenteries (PI. xi., fig. 8). 



Alternating with the mesenteries are large ridge-like vesicular 

 outgrowths from the endoderm. At first sight these appear to be 

 related to the future septa, but against this view it may be urged 

 that the ridges are composed solely of endoderm, the mesogloea 

 scarcely entering into them (PL xi., fig. 10), that there is certainly 

 no trace of calicoblasts or of any ectodermal invagination ; and finally 

 Bourne (1888, p. 28) informs us that the septa of the adult are 

 all entocoelic. 



The endoderm of the mesenteries passes gradually into these 

 ridges ; and at the angles between the mesenteries and the ridges 

 there are numerous Zooxanthellse. A few of the latter may be found 

 in the mesenteries, but they mainly congregate in these twenty- 

 four longitudinal areas ; and it is these which give rise to the 

 twelve pairs of dark longitudinal lines which are so conspicuous 

 in the living larvse. The algse do not always form an unbroken 

 line. 



The mesogloea is an apparently homogeneous jelly-like 

 substance ; whether the ectoderm assists in its formation I cannot 

 say, but it certainly is mainly endodermal in origin. 



The ectoderm of the body-wall presents three characters : — 

 (1) At the aboral end of the body there is a disc-like patch of 

 deep closely set cells. Owing to the absence of Zooxanthellse in 

 the adjacent endoderm this pole appears as a whitish patch in the 

 living embryo, and this forms the seat of attachment of the sessile 

 larva. (2) The ectoderm of the column is composed of a deeper 

 granular or " nervous " layer beneath the ordinary narrow colum- 

 nar cells. Between the latter are an immense number of thread 

 cells, in a few of which the unejected spiral thread can still be 

 seen. (3) At the oral apex the granular layer becomes so much 

 thickened as to practically constitute the whole of the ectoderm. 

 This layer is absent on the pedal disc (PL xi., fig. 10). 



M2 



