94 MARINE ANIMALS OF MASSACHUSETTS BAY. 



upward, meeting in the small dorsal area above, so as to form a 

 spherical outline. Here the ambulacral and interambulacral 

 systems have taken a great preponderance over the dorsal system, 

 and the same is the case with the Holothurians, in which the 

 same structure is greatly elongated, the dorsal system being 

 thus pushed out as it were to the end of a cylinder, while the 

 ambulacral and interambulacral systems run along its whole 

 length. All Echinoderms without exception have ambulacral 

 tubes, even though in some there are no external ambulacral 

 suckers connected with them. 



There is one organ peculiar to the class of Echinoderms, the 

 general structure of which may be described here, since it is 

 common to them all, with the exception of the Crinoids, the 

 anatomy of which is, however, so imperfectly understood, that 

 we are hardly justified in assuming that it does not exist even 

 in that order. This organ is known as the madreporic body ; 

 it is a small sieve or limestone filter opening into a tube or 

 canal ; by means of this tube, which connects with the am- 

 bulacral system, the water from without, first filtered through 

 the madreporic body and thus freed from any impurities, is con- 

 veyed to the ambulacra. In the more detailed account of the 

 different orders we shall see what is the position of this singular 

 organ in each group, and how it is adapted in them all to their 

 special structure. The development of Echinoderms forms one 

 of the most wonderful chapters in the annals of Natural History. 

 Marvellous as is the embryonic history of the Acalephs, including 

 all the different aspects they assume in the cycle of their growth, 

 it is thrown into the shade by the transformations which Echino- 

 derms undergo before assuming their adult condition. This 

 singular mode of development, although it has features recalling 

 the development of Jelly-fishes from Hydroids, is nevertheless 

 entirely distinct from it, and is known only in the class of Echi- 

 noderms. As the whole story is given at length in the chapter 

 on the embryology of the Echinoderms, we need only allude to it 

 here in general terms. We owe the discovery of this remarkable 

 process to Johannes Miiller, one of the greatest anatomists of 

 this century. 



