PEOEESSOB WTVIELE THOMSON ON THE ECHINODEEMATA. 397 
zooids, and the Danish naturalists had an opportunity of examining 
them in detail. The specimens observed were upwards of an inch in 
length. The special natatory organs were fully developed and a young 
starfish was attached to the enlarged posterior extremity. Messrs. 
Koren and Danielssen described in detail and with accuracy the intri¬ 
cate structure of the pseudembryo. According to these observers the 
body consists of a semitransparent structureless substance, imbedding 
multitudes of minute granules and scattered calcareous particles, and 
provided with a delicate layer of longitudinal and transverse fibres, to 
whose contractions the movements of the body and of the swimming 
appendages are due. 
The largest examples of the attached starfish were five mm. in 
diameter. They were of a rich crimson colour and had five short 
arms. The dorsal surface was arched, the ventral fiat. In the 
perisom of the back and sides was imbedded a calcareous annulet 
which gave origin to a multitude of flattened perforated spines. 
Usually five or six of these spines started from a minute calcareous 
turbercle. The sides of the arms were bordered with long spines. 
The ambulacral feet were long and formed a double row. The oral 
plates were large and triangular. 
Messrs. Koren and Danielssen, however, somewhat misconceived 
the relations of the various parts. They regarded the pseudostome 
as a special respiratory orifice for the starfish, while they assigned its 
own proper functions to the pseudoproct. The subsequent observa¬ 
tions of the late Professor Johannes Muller, a naturalist who has 
thrown by his wonderful researches a flood of light on the Em¬ 
bryology of the Echinoderms, are so much more complete that I shall 
content myself with giving his results in detail. 
8. Muller’s first observations were made at Helsingfors, in Sep¬ 
tember, 1847 A The zooids were captured with the towing net in the 
Sound. 
The youngest individuals observed were one-sixth of a line in 
length and perfectly transparent. The dorsal surface was raised and 
keel-shaped, the posterior (anal) extremity round and blunt, the 
dorsal surface curving over on the ventral surface like a hood, 
and ending about the limit of the posterior third of the ventral 
surface in a free border. The anterior extremity was prolonged, 
smooth, and without folds, and beneath it, on the ventral surface, a 
defined buckler-shaped portion passed backwards, ending in a free. 
border opposite the free border of the hood. In a deep transverse 
groove between, lay the pyriform mouth. The oesophagus was 
wide and muscular, the stomach elongated, and passing by a rapid 
contraction into a narrow intestine, which curved rapidly backwards 
and downwards, ending in an anal pore about the centre of the ven¬ 
tral curve of the posterior hood. The surface of the zooid was 
entirely covered with cilia, and by their agency it swam rapidly in 
the water. 
* “ Ueber die Larven und die metamorphose der Echinodermen.”—Berlin, 1849. 
