LECTURES ON EMBRYOLOGY. 



25 



troducing a natural classification among these an- 

 imals ? 's a further question which lays in my 

 plan; as these embryonic investigations were trac- 

 ed from the beginning with reference to the classi- 

 fication of the animal kingdom in relation to the 

 order of all types, when compared with the chang- 

 es which embryos undergo. 



Among Echinoderms the investigation of struc- 

 ture has already settled the classification to this 

 extent,that they have been divided into three fam- 

 ilies,Holothurians,the tubular ones. Plates XIV & V, 

 & V; Echini,the spherical ones, Plate XIII; and the 

 Asterians, (Plate VIII,) the star-shaped ones : but 

 from this general arrangement there is still a con- 

 siderable distance to the perfect fixation of the or- 

 der of succession of genera in all their details. The 

 various arrangements which have been proposed 

 have been influenced by the various states of our 

 knowledge. The improvements in the classifica- 

 tion of Echinoderms have been greatly advanced 

 by the knowledge of the Crinoids, which are 

 universally placed in the lowest rank among those 

 animals, from their resemblance to Polyps. When 

 their structure was ascertained. the knowledge thus 

 acquired, did not modify the position which was 

 assigned to them when not yet sufficiently known. 

 The knowledge of the change in the growth of one 

 Crinoid, the Comatula, has indeed influenced more 

 the classification than the knowledge of their struc- 

 ture. The free star-fishes are placed next to the 

 Echini and above all the Holothuriae. Among Ech- 

 ini we have some in which the mouth is central 

 and the alimentary canal ends on the margin ; and 

 there are others in which the alimentary canal 

 enda on the two extremities of the body, as seen 

 here, (Plate VI fig. B.) thus forming a transition to 

 the worm-like form, they indeed begin to be re- 

 lated to the Holothurite (Plate XIV) and will rank 

 higher. 



[PLA.TE VI SEA URCHINS ] 



Structure and embryonic growth have satisfied us 

 thus far. But why should we not venture to go fur- 



ther,and make use of the order of succession of these 

 types, in order to ascertain all their relations ? The 

 Crinoids which have been 'described as fossils, are 

 exceedingly numerous. Here are figured several 

 forms, to which I have not yet alluded. Plate 

 XVII, fig. C, is a genus called Caryoirinus. Here 

 is another, which occurs also in old strata, (Fig. B) 

 called Pentreraites; and here (Fig. D) one which oc- 

 curs in deposites of the coal period, called Echino- 

 crinus. 



In Plate XVII, fig. C, we have a spherical body, 

 like an Echinus, with a stem as in Crinoids, but the 

 plates are not yet ranged in regular rows (Fig. C), 

 but alternate irregularly ; there are not yet rows 

 for the pores distinctly circumscribed, but only at 

 irregular intervals, and few of them. This form, 

 as also the Sphoronites are the most primitive Cri- 

 noids, and they correspond somewhat in structure 

 to the earliest condition which we observe in Echi- 

 ni, and which we observe also in the youngest 

 stage of the star fish. 



Here is one (Plate XVII, fig B) in which we have 

 a mere star' fish-like form ; the sphere is in its full 

 condition of development ; and here we have one 

 which would seem to be a common sea-urchin, 

 (Fig. D.) But on comparing both (Plate XIII, fig. 

 C) they are found widely different. In Echinus 

 (Plate XIII, fig. C) there are two rows of perfora- 

 ted and two of imperforated plates, while in Echi- 

 nocrinus (Plate XVII, fig. D) there are four rows 

 of imperforated plates, and the animal is really a 

 crinoid. and not a sea-urchin. This (Fig. D) has a 

 stem : that (Plate XIII, fig. C) has not. The Cri- 

 noids are found in ancient geological strata in the 

 middle geological ages are those of (Plate VII). 

 Free Star -fishes begin later in the geological for- 

 mations. The Comatula or free Crinoids are again 

 later (Plate I, tig A). The Echini appear long af- 

 ter the families of Crinoids and free star-fishes 

 have been introduced upon our globe. We have 

 not yet one of the spherical Echinoderms before 

 the deposition of the red stone or the Marchalkalk 

 of Germany. And those spherical Echini or Cida- 

 ris are the earliest ones, (Plate VI, figs. D and E.) 

 Next we have such as have a central mouth, and 

 in which the alimentary canal ends laterally. 



And at a later epoch those which have an elong- 

 ated body (Plate VI. fig. B.) The first epoch in 

 which elongated Echini appear is in the chalk de- 

 posit. 



When there was not yet one free starfish, there 

 were only Crinoids on earth. And what sort of 

 Crinoids had we ? Not such as already resembled 

 common starfishes (Plate VII.), but which resem- 

 bled the lowest stage of growth of these animals, 

 when they are still without arms (Plate XI. fig. E.) 

 with irregular arrangement of their plates (Plate 

 XVII. fig. C.) Next we have such which assume 

 the.shapeof the star-fish, (Plate XVII. fig. B.) but 

 are stilll Crinoids resting on stems with few irregu- 

 lar plates, but in which holes are arranged in a re- 

 gular star above. And next we have Echinocrinus, 



