18 



PROF. AGASS1Z S 



er types produce their young during winter. But 

 on considering what may be the cause of their 

 egcs being deposited at this seaHon,we can suppose 

 it is owing to the fact, that during this epoch the 

 water is less changeable in its temperature and will 

 admit of a more uniform growth of animal life 

 than during the spring and summer. All animals 

 of low temperature or whose temperature is deep- 

 ly influenced by the surrounding medium, in op- 

 position to the higher organized ones, seem in- 

 deed to develope more naturally during the cold 

 period of the winter, when the possible Changes 

 are only slight, undulating about the freezing 

 point, from about the temperature of the greatest 

 density of water to that of the freezing point it- 



self, that is between 32 Q and 38 & . The limits of va- 

 riation of the temperature of water being so very 

 slight under such circumstances, we can conceive 

 that these low animals are more likely to devel- 

 ope regularly than under the changing influences 

 of spring and summer; when along the shores the 

 influences are extremely variable and might kill 

 so delicate animals which have no means to main- 

 tain a temperature of their own. 



In my next lecture, I shall compare these em- 

 bryonic changes with the perfect state of the vari- 

 ous Echinoderms of the present creation, and with 

 the perfect state of the numerous fossils of this 

 class which have been discovered by geologists in 

 the successive deposits of former ages. 



LECTURE III. 



I have shown, in one instance, the development 

 cf the star-fishes, as observed on these shores 

 during winter. It was mentioned, that from a 

 spherical form there was gradually a flattened disk, 

 a hanging peduncle, developed, out of which after- 

 wards arose a pentagonal form, which was finally 

 changed into a regular starfish, with the structure 

 of the full grown animals of that class. These 

 changes have been traced from the beginning of 

 the formation of the germ in the egg, when they 

 are protected by the mother who takes care of 

 them, carrying them about. At no period of this 

 development were the young star-fishes observed 

 swimming free. There can, however, scarcely be 

 any doubt that the young observed on the Norwe^- 

 gian shore were free. The observations of Sara 

 can the less be doubted in that points as similar 

 moving animals, which were afterwards ascertain- 

 ed to have been the star-fish and other Echino- 

 derms, have been discovered by the investigations 

 of Professor Johannes Muller, of Berlin. This 

 minute and leaded investigator described, several 

 years ago, a small animal as a new type in the ani- 

 mal kingdom, which he could not refer to any 

 class, nor to any family. It was a paradoxicon by 

 its form and its peculiarities, and he called it Plu- 

 teus Paradoxus. It is a transparent mass, support- 

 ed by several diverging sticks, surrounding an 

 internal cavity (PL XII, A) and moving free upon 

 the surface of the water* I have not dared to have 

 them shaded in my diagrams, in order to increase 

 the distinctness of the forms ; and only give these 

 slight outlines as they are figured by Muller. In 

 this condition, that animal is bi-lateral as seen from 



above. At the two extremetieS of the longitudi- 

 nal axis, are two appendages 5 these appendages 

 are the stems which project laterally in Figs. A 

 and B, pi. XII., they being the anterior and poste- 

 rior ends of the longitudinal axis. Between thesej 

 you see one shorter pair on one side, and on the 

 other side another pair, which hang lower down* 

 [PI. XII., figs. A. B.] These two pairs of appen* 

 dages are indeed not equal in length. One pair on 

 one of the sides hangs lower down than the other 

 pair. Between those six supports, united by a gel* 

 atinous solid mass, there is an inner cavity, as 

 seen in the figures quoted. The side of the longer 

 ends has lateral projections, so that, in fact, there 

 are eight prominent sticks diverging from the sum- 

 mit of this curious being. No further structure 

 was observed in the first year by Mr. Muller. He 

 only ascertained they moved free in all directions^ 

 sometimes rising forwards and sometimes revolv- 

 ing in different directions. 



These movements were performed by vibratory 

 cilia, which are minute fringes extending all 

 around the edges of the frame, and which are also 

 grouped on the summit of the animaL These 

 fringes are microscopic. They form a swollen 

 edge round the whole of these dentations, extend- 

 ing all round the edge of these stems. (Plate XII, 

 fig. B.) What this being was, could not be ascer- 

 tained. It had been observed in the Northern 

 Sea, in thousands and thousands, and could not be 

 referred to its proper class. Whether it was to be 

 considered a medusa or a polyp, or whether it 

 was the germ of some other animal, could not fos 

 ascertained. 



