10 MARINE ANIMALS OF MASSACHUSETTS BAY. 



and slightly spreading around the margin. (Fig. 2.) The upper 

 edge of this bag turns in so as to form a sac within a sac. (Fig. 



6.) This inner sac, s, is the stomach or 

 digestive cavity, forming a simple open 

 space m the centre of the body, with an 

 aperture in the bottom, h, through which 

 the food passes into the larger sac, in 

 which it is enclosed. But this outer 

 and larger sac or main cavity of the 

 body IS not, like the inner one, a simple open space. It is, on the 

 contrary, divided by vertical partitions into a number of distinct 

 chambers, converging from the periphery to the centre. These 

 partitions do not all advance so far as actually to join the wall of 

 the digestive cavity hanging in the centre of the body, but most 

 of them stop a little short of it, leaving thus a small, open space 

 between the chambers and the inner sac. (Fig. 1.) The eggs 

 hang on the inner edge of the partitions ; when mature they 

 drop into the main cavity, enter the inner digestive cavity through 

 its lower opening, and are passed out through the mouth. 



The embryo bears no resemblance to the mature animal. It is 

 a little planula, semi-transparent, oblong, entirely covered with 

 "sdbratile cilia, by means of which it swims freely about in the 

 water till it establishes itself on some rocky surface, the end 

 by which it becomes attached spreading slightly and fitting 

 itself to the inequalities of the rock so as to form a secure basis. 

 The upper end then becomes depressed toward the centre, that 

 depression deepening more and more till it forms the inner sac, 

 or in other words the digestive cavity described above. The open 

 mouth of this inner sac, which may, however, be closed at will, 

 since the whole substance of the body is exceedingly contractile, 

 is the oral opening or so-called mouth of the animal. We have 

 seen how the main cavity becomes divided by radiating partitions 

 into numerous chambers ; but while these internal changes are 

 going on, corresponding external appendages are forming in the 

 shape of the tentacles, which add so much to the beauty of the 

 animal, and play so important a part in its history. The ten- 

 Fig. 6. Vertical section of an Actinia, showing a primary (17) and a secondary partition g' ; mouth, 

 t tentacles, s stomach, // reproductive organs, b main cavity, c openings in partitions, a lower floor, or 

 foot. ' 



