142 THE ANATOMY OF INVERTEBRATED ANIMALS. 



bryo is a bilaterally symmetrical cylindrical body, with a cen* 

 tral canal, the future gastric sac ; and, communicating there- 

 with, a bilobed enterocoele, whicn separates the central canal 

 from the body-wall. In fact, in principle, it resembles the 

 early condition of the embryo of a Ctenophore, a Brachiopod, 

 or a Sagltta. 



Another pair of mesenteric processes now makes its ap- 

 pearance in the larger chamber A', and cuts off two lateral 

 chambers, B, B, which lie between these secondary mesenteries 

 and the primary ones. In this state the enterocoele or somat- 

 ic cavity is four-chambered (A-^t? A'). Next a third pair 



of mesenteries appear in the smaller chamber (A), and divide 

 it into three portions, one at the end of the oral axis (A), 

 and two lateral (C, C). In this stage there are therefore six 



A p-^T3 A' ) ; but almost immediately the number 



is increased to eight, by the development of a fourth pair of 

 mesenteries in the chambers B, B, which thus give rise to the 

 chambers D, D, between the primitive mesenteries and them- 

 selves. The embryo remains in the eight-chambered condition 



(a p/-^T-) -D A' ) for some time, until all the chambers and their 



dividing mesenteries become equal. Then a fifth and a sixth 

 pair of mesenteries are formed in the chambers C, C, and D, D ; 

 two pairs of new chambers, E and F, are produced, and thus the 



Actinia acquires twelve chambers ( A p tt'^f t) t^ A'), five 



of which result from the subdivision of the smaller primary 

 chamber, and seven from that of the larger primary chamber. 

 The various chambers now acquire equal dimensions, and the 

 tentacles begin to bud out from each. The appearance of 

 the tentacles, however, is not simultaneous. That which pro- 

 ceeds from the chamber A' is earliest to appear, and for some 

 time is largest, and, at first, eight of the tentacles are larger 

 than the other four. 



The coiled marginal ends of the mesenteries appear at 

 first upon the edges of the two primary mesenteries ; then 

 upon the edge of the fourth pair, and afterward upon those 

 of the other pairs. 



For the further changes of the young Actinia, I must 

 refer to the work cited. Sufficient has been said to show that 

 the development of the ActinicB follows a law of bilateral 

 symmetry, and to bring out the important fact that, in the 



