LECTURES ON EMBRYOLOGY. 



IPLAXE XX POLYPI ACTINIAE, CORYNJE, SYN- 



CORYN^E, PODOCORAN.E.] 



one to several inches in length when fully ex- 

 panded ; it consists of a membranous sac, as in all 

 Polypi, with numerous tentacles round the upper 

 extremity, and contains within, another sac, open- 

 ing above between the several rows of tentacles, 



[PLATE XXXII POLYPI ACTINIA.] 



In this drawing (Plate XXXII, fig. B) you notice 

 the whole structure in a vertical section of the an- 

 imal, in which the relations between the different 

 parts and their interior cavities are at once seen. 

 You notice the external walls of the animal, and 

 the rows of tentacles forming the upper outline. 

 And from the centre, where there is a large open- 

 ing which must be considered as the mouth, hangs 

 down a thin sac, suspended within the cavity form- 

 ed by the external arms and the surrounding thick 

 envelop of the bodj r . 



This sac is a stomach; it is maintained in its po- 

 sition by internal nuliating membranes, extending 

 all around the mouth and stomach and uniting with 

 the external envelop so as to divide the interven- 

 ing space into many chambers. There are also 

 shorter folds which penetrate from the external 

 walls towards the centre, so that the space between 

 the stomach and the lateral walls is not one con- 

 tinuous cavity, nor uniformly divided into equal 

 chambers, but it is a cavity divided and subdivided 

 anto wider and narrower spaces by partitions which 

 extend either entirely across the cavity surround- 

 ing the stomach, or only partly into it, thus form- 

 ing imperfect chambers; all the them, however, 

 remaining connected by the open space which is 

 left free of divisions below the stomach. Here is 

 a diagram [Plate XXXII fig. A.] in which the ani- 

 mal is represented as divided horizontally, and in 

 this horizontal section you see the cavity of the 

 stomach forming one great whole in the centre 

 and the partitions which extend from the external 



walls towards the centre either reaching the walls 

 of the stomach or not, from the intervening septa, 

 But these are not all equal. There are some of the 

 partitions which reach half way towards the stom- 

 ach others whieh reach two-thirds of the way 

 and others still which reach most of the distance. 

 Below [Plate XXXII fig. B.] we see them as they 

 present themselves upon a vertical cut. From the 

 external surface something of those partitions is 

 already seen. The vertical striae noticed [Plate 

 XX fig D ] are the external points of attachment 

 of the fleshy partitions upon the external envelop 

 of the whole body, and they extend high up into 

 the margin from which the tentacles arise, And 

 indeed on close examination it will be seen that 

 one tentacle arises always between two partitions ; 

 so that a tentacle is, as it were, a radiating prolon- 

 gation of the main cavity of the body, extending 

 like the finger of a glove from each of the divided 

 spaces upwards. You see this [Plate XXXII fig. 

 BJ where the tentacles show plainly their connec- 

 tion with the main cavity, and where the divisions 

 are as numerous as the tentacles. 



These partitions are muscular fibres, and by their 

 contractions they can shorten the animal. Sup- 

 pose these vertical partitions to be at once contrac- 

 ted, the animal, instead of forming a vertical cylin- 

 der, becomes depressed. (Plate XX fig. E J And 

 as there are muscular fibres around the whole 

 body, the tentacles cae be drawn in, and the upper 

 fibres, contracting more and more, may entirely 

 conceal the tentacles and form such hemi spheri- 

 cal bodies as are observed in these diagrams. 

 [Plate XX fig. G.J 



Between these partitions, by very careful investi- 

 gation, small holes can be discovered, arranged in 

 vertical series {fig, D). The use of these tubes is 

 not yet fully ascertained. I shall have an oppor- 

 tunity to refer to them again. 



But I would mention, further, that the mouth 

 (Plate XX , fig. F) is not a simple circular hole on 

 the summit of the animal, but presents lateral 

 folds upon a longitudinal fissure. At first sight, 

 when seen from above, the inner membrane of the 

 Actinia stretched between the tentacles seems to 

 form a circular mouth (Plate XXXIII, fig. A); but 

 on close examination, it will be noticed that it is 



[PLATE XXXIII. -POLYPI YOUNG ACTINIA.] 



