46 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



GENERAL CONSIDERATION OF THE CHEILOSTOMATA. 

 ENDOCTST AND ECTOCYST. 



The skeletal portion of the bryozoid is lined interiorly by a very fine epi- 

 thelial membrane called the endocyst. This endocyst is the essentially living part of 

 the bryozoan. It grows without cessation by the proliferation of its elements, and 

 it secretes the different cuticles. It emits buds of like characteristics but which 

 engender by successive differentation the various organs of the bryozoid. 1 



The first differentation, almost immediate, indeed, is the ectocyst- (epitheca of 

 Harmer. outer membrane, frontal membrane of Waters). This is generally thin, 

 so thin sometimes that its presence is often doubtful: 3 it has no secreting power. 



The calcareous or chitinous secretion forming the " zoarial skeleton " occurs 

 between the ectocyst and the endocyst. This is the only part capable of fossiliza- 

 tion. The study of the relations between the skeleton and the living organs of the 

 zoarium forms the essential object of study of paleontologic bryozoology. 



The second differentiation of the endocyst is the formation of the mesenchyme 

 and of its successive derivatives polypides. leucocytes, etc. 4 This study is in the 

 domain of zoology exclusively. 



The difference of orientation in the proliferation of the endocystal elements is 

 most important. In the Anasca this proliferation occurs only laterally. In 

 correlation early on the budding parts of the zoarium. the endocyst divides in two 

 and provokes a corresponding division of the ectocyst, thus engendering the hydro- 

 static apparatus called the hypostege (fig. 3). 5 



In the Ascophora the proliferation occurs in two ways lateral and frontal. 

 In correlation the endocyst develops on bth sides of the zoarial skeleton, owing 

 to the emission of frontal buds (fig. 3). Thus in the Anasca the buds are arranged 

 side by side, while in the Ascophora they are placed over each other. 



ZOARIAL BUDDING AND FORMATION" (IF SKELETON. 



The buds of the endocyst are as noted above, lateral or frontal. They evidently 

 secrete lime and occasion secondary calcification. 

 The lateral buds are: 



1. Little developed and leave in fossilization the lateral punctations (=parietal 

 dietellae) of the Adeonidae. 



C 2. More developed, they form the avicularia. the interzooecial vibracula (Ade- 

 onidae, Lunulariidae, Onychocellidae). 



1 1900. Calvet, Contributions a 1'historie des bryr>zn:iirps .-rtoi.rix-tes marins, Travaux de I'institut 

 Zoologie de 1'Universite' de Montpellier, new ser., Menioire No. S, p. 170. 



2 1SS6. Joliet, Recherches sur la blastoggnese. Art-hives de Zoologie experimental et generale, ser. 2, 

 vol. 4. r>. (.;.">. 



3 The word ectocyst is here employed in a different lueauing from that of other authors who designated 

 as the " ectocyst " that part of the bryozooid which we call the skeleton. 



1 1900. Calvet, Travaux de 1'Institute Zoologie de 1'Vnm-rsitt fit- de Montpellier, p. 239. 

 = 1900. Calvet, Idem, p. 160. 



