528 E. A. MINCHIN. 



protoplasm, the granules might exercise a great influence on 

 internal movements of the protoplasmic framework, such as 

 those which result in contraction in one or another direction 

 (1892, pp. 207—209; English translation, pp. 323—327). The 

 distribution of the granules in the dermal layer favours this 

 view of their function, since they are most abundant in the 

 very contractile porocytes, of which any specially contractile 

 organs — sphincters or sieve membranes — are formed, while in 

 the spicule-secreting cells they rapidly disappear. An excep- 

 tion to this is apparently to be found in the case of the gastral 

 actinoblasts, which, however, according to Lieberkiihn (1865, 

 p. 737), are retractile, a fact that would explain the persist- 

 ence of the granules in them. It is noteworthy also that 

 while the dermal layer of the Clathrinidse is remarkable 

 both for its richness in granules and for its contractility, the 

 corresponding layer in the non-contractile Leucosolenias is 

 very clear and free from large granules. 



The theory that the granules aid in the contraction is, how- 

 ever, devoid of any actual basis of experiment or observation, 

 and is to be regarded merely as a possible hypothesis, which is 

 at least worth testing. 



III. Historical : Earlier Observations upon Calcareous 



AND other Spicules. 



(a) Observations upon Spicule Formation. — Direct 

 observations on the origin and growth of the spicules in cal- 

 careous sponges are few and far between in the literature of the 

 group ; confident assumptions, on the other hand, with regard 

 to the way in which it is supposed a priori that the spicules 

 should and must arise, are to be found in abundance. In the 

 almost total absence of facts, the foundation for the current 

 beliefs concerning spicule formation in the Calcarea is to be 

 sought partly in deductions from theory, partly in analogies 

 from the well-established facts as to the origin of siliceous 

 spicules. It is best, therefore, to begin by a brief survey of 

 what has been ascertained in non-calcareous sponges. 



