DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 779 
question presents serious difficulties under the theory of natural selection, for it is 
impossible to see how the occasional appearance of minute calcareous nodules in the 
tissues can originally have been of any use to the organism. 
Everyone is familiar with the formation of crystals of lime salts in the interior of 
vegetable cells, where they perform no function as supporting organs, and are 
apparently mere by-products of the activity of the protoplasm. In this respect the 
entodermic spicules of Renilla resemble the deposits in many vegetable cells ; for 
they are of no use to the colony as supporting organs, and unless we consider their 
present condition as having been acquired through degeneration, they must originally 
have been developed without reference to such a function. From the analogy of the 
deposits in vegetable cells, and in the entoderm cells of Renilla, it seems not im- 
probable that the ectodermic spicules of Renilla had originally no function as 
supporting organs, having been formed simply as by-products of the activity of the 
protoplasm under peculiar conditions, such, for instance, as a superabundance of lime 
salts in the water. If, however, calcareous nodules once made their appearance in 
any considerable quantity in the tissues, they might serve as supporting organs, and 
be developed through natural selection to almost any extent. They might thus 
attain the great size and functional importance of the ectodermic spicules of Renilla 
or other Pennatulids, or by agglutination come to form a compact skeleton as in 
Tubipora. 
It is remarkable to find so wide a difference between the skeletons of Aleyonaria 
and Zoantharia, as must exist if Kocu’s recent conclusions as to the skeleton of 
Asteroides calycularis are well founded ;* and it seems probable that the skeleton 
has been quite independently acquired in the two groups. The present considerations 
will, of course, apply to the Alcyonarian skeleton only. 
§ 13. Development of the muscular system. 
The larva of three days is very changeable in form (figs. 104, 105), showing that 
contractile elements have made their appearance; and careful examination of 
specimens rendered transparent by reagents reveals the presence of numerous short 
delicate unstriated muscle-fibres underlying the ectoderm. These are found to have 
a definite and constant arrangement, which will be described before considering the 
histology of the tissue. 
a. Distribution. 
The muscle-fibres are from the first arranged in two systems, viz. : a layer of longi- 
tudinal fibres, and a layer of circular fibres, which ultimately come to le outside the 
former. The circular fibres first appear in the posterior half of the body in a broad 
* Mittheilungen aus der Zool. Station zu Neapel, Band iii., Heft iii., 1882, pp. 284-292. 
