METAZOA PORIFERA. 7 1 



work is accomplished is represented at the base of the 

 collar by a black line. 



Several authors describe the endoderm in this genus as 

 many layered, but Minchin proved that this appearance 

 is wholly due to contraction. When fully expanded the 

 endoderm has only one layer, but when contracted it is as 

 shown in fig. 8 ; fig. 9 is a portion of the endoderm from 

 fig. 8 more highly magnified. 



The genus Sycandra is one of the most differentiated 

 of the calcareous sponges. Some of the species, like 

 Sycandra (=Sycon) raphanus Hkl. (No. 65), are single, 

 while other species, like Sycandra arborea Hkl., form col- 

 onies. The egg and spermatozoon in Sycandra raphanus 

 Hkl., are transformed cells of the mesoderm. The fertil- 

 ized egg (PI. 64, fig. i) possesses a nucleus and is capable 

 of creeping amoeboid movements. In fig. 2 the nucleus 

 has divided. Fig. 3 shows the first cleavage or furrowing 

 stage from above, and fig. 3 a the same from the side. 

 Fig. 4 shows four cells, fig. 5 eight cells still lying in pairs ; 

 fig. 5 a the same from the side; fig. 6 sixteen cells, fig. 6a 

 the same from the side ; in fig. 7 a large number are rep- 

 resented. This repeated division gives rise to a hollow 

 sphere, the wall of which is formed by a single layer of 

 celis. PI. 64, fig. 8, is the blastula with its eight dark 

 granular cells surrounding a basal opening, and fig. 9 is a 

 further developed, entirely closed blastula. In fig. 10 the 

 embryo has become differentiated into halves unlike each 

 other, for which reason it is known as an amphiblastula. 

 This is probably a modification of the primitive blastula 

 already described, and if so it is a secondary and more 

 specialized form. The granular cells have increased in 

 number, and are at the broader end, while flagellated cells 

 are at the smaller end. In this condition the embryo 

 leaves the parent. Fig. 1 1 is a more advanced stage in 

 which the flagellated layer has become flattened ; in fig. 

 12, it is still more depressed, and in fig. 13 has disappeared 

 within (see note p. 66). The larva settles mouth downward 



