46 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
GENERAL CONSIDERATION OF THE CHEILOSTOMATA. 
UNDO CYST 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 2 
The first differentation, almost immediate, indeed, is the ectocyst 2 (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 endocj^st is the formation of the mesenchyme 
and of its successive derivatives — polvpides, 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 both 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 OF 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. 
2. More developed, they form the avicularia, the interzooecial vibracula (Ade- 
onidae, Lunulariidae, Onychocellidae) . 
1 1900. Calvet, Contributions a l’historie des bryozoaires ectoproctes marins, Travaux de l’institut 
Zoologie de l’Universitt; de Montpellier, new ser., Memoire No. 8, p. 170. 
2 1886. Joliet, Recherches sur la blastogen&se, Archives de Zoologie experimentale et gengrale, ser. 2, 
vol. 4, p. 65. 
3 The word ectocyst is here employed in a different meaning from that of other authors who designated 
as the “ ectocyst ” that part of the bryozooid which we call the skeleton. 
4 1900. Calvet, Travaux de l’lnstitute Zoologie de l’Universit6 de de Montpellier, p. 239. 
6 1900. Calvet, Idem, p. 166. 
