564 SUMMARY OF CURRENT RESEARCHES RELATING TO 



bach (1866), for this name is pre-occupied. The original generic de- 

 scription is as follows : — Vermicular, perforate sclerenchyma forming 

 encrusting masses surrounded by a thin epitheca. Cells polygonal, 

 moderately deep, with a strongly-developed spongy columella. Walls 

 massive. Spines occur at the corners of the cells. Septa strongly 

 crenulated ; descending abruptly to the calyx floor. It is pointed out 

 that Tumularia differs from Protaraea in having eight or sixteeen septa, 

 not twelve ; an essential columella instead of a parietal columella or none 

 at all ; and broad petallif orm septa, not lamellar. The author calls it an 

 Alcyonarian, and places it among the Heliolitidifi. 



Porifera. 



Factors in Evolution of Sponges.* — Arthur Dendy illustrates the 

 suitability of the sponge phylum for the study of evolutionary processes. 

 Many skeletal features can be interpreted as adaptive, others seem to 

 nave' no sort of relation to the requirements of the organism. Many 

 adaptive changes have been determined largely by mechanical conditions, 

 e.g. in relation to the canal system and the gastral and dermal cortex. 

 The forms of spicules are often adaptive, and " it really seems as if the 

 sponge were able to do anything that may be required with the inherited 

 material at its disposal, to convert a calthrops into a pitchfork, a grapnel, 

 or an armour plate as occasion demands." On the other hand, the 

 extraordinary forms assumed by so many of the microscleres of the 

 Tetraxonida, all derivable from the primitive tetract, do not seem to 

 have any adaptive significance. The same may be said of the chehe of 

 Desmacidonidte. The reason for the existence of the divers forms of 

 chelae is a complete mystery ; "we can only say that they must l)e, to 

 use a mathematical expression, functions of heritable modifications in 

 the constitution of the protoplasm of the germ-cells, the nature of 

 ■which is as yet beyond reach of investigation, and that they are perhaps 

 correlated with other characters which are of real value to the sponge." 

 They probably originate as mutations. Peculiar characters, not of the 

 nature of convergence, may occur in widely separated families. This 

 may be illustrated by the discontinuous distribution of trichodragmata 

 amongst the Tetraxonida. The trichodragma is a bundle of short, 

 hair-like spicules, apparently all originating in one and the same rnother- 

 cell. A similar phenomenon is observable in the case of the microxea 

 amongst Calcarea. "Discontinuity in the present distribution of 

 trichodragmata and microxea almost certainly implies discontinuity in 

 origin. Probably these spicules have arisen suddenly, and on many 

 occasions, as the result of some unknown change in the constitution of 

 the germ-plasm. It is, no doubt, more a chemico-physical than a 

 biological phenomenon, and that, I believe, is true of all mutations." 

 The author holds that the transformations of spicules suggest the 

 existence of definite factors in the germ-plasm. The same view is 

 suggested by the simplification which sometimes results by the complete 

 dropping out of certain types of spicule. The Epipolasidse of Sollas do 



* ' Journ. Quekett Micr. Club, xiii. (1916) pp. 27-46. 



