LECTURES ON EMBRYOLOGY". 



15 



have an envelope which is entirely separated 

 from the internal organs, forming a covering, which 

 is either hard, leathery and strong, or a firm coat, 

 consisting of numerous calcareous plates united 

 together, or connected together in a movable 

 way. These external coverings are not like a 

 shell resting on the soft parts, but they form intrU 

 cated connections with all the different systems of 

 organs, although these be distinctly separated 

 from the external envelope. For this purpose they 

 are pierced by numerous holes of various kinds, 

 indeed the connexion of this external covering 

 with the internal frame is manifold. The mouth 

 again, which is always toward the centre of the 

 animal, is also only an opening in the middle of 

 the disk, and upon its edge are various movable 

 parts performing the functions either of teeth or 

 tentacles, by which the food is seized, In another 

 position, frequently opposite the mouth, there are 

 other apertures by which the ovaries discharge the 

 eggs, and little holes in which eye -like organs are 

 placed. The organs of respiration which admit 

 water from outside are either in the general cavity 

 of the body, or situated more externally, round the 

 mouth, or on the sides of the animal. 



There is in these lower animals a closer connex- 

 ion between their inner cavity and the surrounding 

 media than in any of the higher classes. The 

 water rushes freely into the body through innu- 

 merable pores and fills its cavity, Some of these 

 tubes assume a very peculiar arrangement in 

 -achinoderms, and become simultaneously subser- 

 vient to locomotion. As this apparatus is one of 

 the first to appear.in the young, let me allude to 

 its structure as we observe it in the starfish. Here 

 are the different rays [Plate VIII fig, B] project- 

 ing from the centre. There is a sac projecting 

 in the main cavity of the body a stomach and 

 frr.m this stomach we have appendages projecting 

 into the rays, to which a kind of liver is annexed, 

 and filling for the most part the cavity of the rays. 



IPLATE 



In the figures [Plate IV, figs. E, F] in which the 

 starfish is cut vertically, the sacks extending from 

 the stomach, with the liver attached to them, are 

 ia tfeeir natural position The nervous sys- 



tem forms a ring all around the walls of the open- 

 ing leading to the stomach ; and there are ner- 

 vous threads arising from this central ring to each 

 of the rays, and extending in five different direc- 

 tions to their extremity. And at the end of each 

 ray there is a colored dot protected by a hard 

 shield. This colored dot has been ascertained to 

 resemble the lowest form of eyes. 



The solid frame which protects the whole ani- 

 mal consists of various little plates, [Plate VIII. ] 



They are numerous on each side of the rays, and 

 there is another one at the end, and it is below this 

 last one that the eye is placed. Those solid plates 

 on the two sides unite with many others placed 

 transversely to form the lower surface, and alter- 

 nating with each other. Between these transverse 

 plates are the holes for the tubes mentioned be- 

 fore. At the end is the odd plate. 



These tubes are seen here hanging down [Plate 

 IV. fig. E.j. They communicate inside with small 

 vesicles, to which minute tubes lead, communicat- 

 ing with larger tubes, which extend,along all the 

 rays, one for each ray, arising from a circular tube, 

 which surrounds the opening of the stomach. And 

 the whole apparatus communicates with another 

 tube, which penetrates from the dorsal surface 

 downwards, having its opening shut by a perforated 

 plate called the madreporicbody, which in starfish^ 

 es is always seen in the angle between two of the 

 rays ; so that we have here an hydraulic apparatus 

 of a very complicated nature. Indeed, from the 

 upper surface of the starfish, where the little seive 

 through which the water penetrates is situated, 

 there is an uninterrupted communication to the 

 circular tube around the mouth, from which five 

 tubes branch out, one to each of the five rays ; and 

 from these, they open to the vesicles, and thence 

 penetrate into the tubes. But the water can enter 

 the vesicles through the external lower tubes, fill 

 the circular tubes, and pass out the other way 

 through the madreporic body. This apparatus is 

 subservient to various functions. In the first place^ 

 the lower tubes serve as a walking apparatus ; the 

 animal being fixed and creeping by the contraction 

 of the tubes, and again water being introduced into 

 the vesicles upon which are spread numerous little 

 blood vessels, and the water acting upon these 

 blood vessels modifies the blood, and gives it the 

 peculiar character necessary to perform its func- 

 tions, constituting a peculiar kind of respiratory 

 system. The minute holes spread over the whole 

 surface of the body, serve simply to fill the general 

 cavity with water* 



The heart is placed along the calcareous tube 

 whteh arises from the madreporic body, and the 

 blood vessels form circular rings around the en= 

 trance of the stomach, from which and to which 

 the radiating arteries and veins move. 



Another apparatus which is very voluminous ift 

 starfishes is the ovary. 



There is such an organ in each ray, concealed be- 

 tween the appendages of the stomach, which opes 



