Hyatt.] 138 [March 5, 



the suppression of the formation of complete buds. Thus the 

 direct growth of the individual has built up spongozoons which 

 may be called massive or branching individuals, but are in no 

 sense colonies, except in the forms with a thin mesoderm, Ascones. 

 The characteristics of the middle layer are so exceedingly primi- 

 tive that we have called it mesenchyme. It is the seat of the 

 reproductive, as well ns assimilative cells in all except the lowest 

 forms, Ascones, thus presenting a strong contrast with the meso- 

 blastic and mesodermic tissues of higher forms. 



The peculiarities of the type are still more marked in the 

 young. The earliest stage, as in all Metazoa, is amoeboidal, but 

 in the next stage the monoplaculate form of embryo is prevalent, 

 as might have been anticipated, in the Calcispongiae as the most 

 generalized structures among the Metazoa. The more concen- 

 trated forms of the ovum, the morula, amphimorula, parenchy- 

 mula and gastrula, are not peculiar to the Porifera ; but in the later 

 stages the Cinctoplanula appears, and at once enables the ob- 

 server to distinguish any form in which it is present as a sponge, 

 and this is followed by the equally characteristic ascula and ara- 

 pullinuln. There are unquestionably type-larval stages in most of 

 the branches of the animal kingdom, and time and fuller knowledge 

 will probably bring embryology and adult structures into agree- 

 ment with each other. 



Not only can we distinguish a sponge by the Cinctoplanula, 

 Ascula and Ampullinula, and the Ilydrozoa by means of the 

 Hydroplanula and Actinula, and the Actinozoa by the formation of 

 the fleshy septa and characteristics of the Gulinula, but it has been 

 long recognized that this was practicable in the last two branch- 

 es, as well as among Crustacea in which the Nauplius is preva- 

 lent, among Mollusca in which the Veliger is typical, among 

 Echinodermata in which the Pluteus prevails, and among Verte- 

 brata in which the test of affinity is the notochordal stage. In all 

 of these branches, except the last, there are groups in which the 

 typical embryonic or larval stage is absent, and in some certainly 

 this absence can be accounted for as due to the law of concentra- 

 tion. The suppression of the Cinctoplanula in Sycandra, the 

 absence of the nauplius in the Astacidae, and of the Veliger in the 

 Cephalopoda are all examples of highly specialized type which 

 concentrate or suppress the characteristics of the type-larval stage 



