CCELENTERATA : AIOLLUSCOTDEA 943 



— a free-swimming sexual generation, which, however, sometimes 

 remains attached to the parent. Many Hydrozoa secrete a horny 

 or chitinous envelope, which ends in many cases in cups or li\dro- 

 theccc. In the fossil graptolites these horny structures alone are 

 preserved, as compressed carhonaceous films. In other cases 

 (Hydrocorallines) a calcareous structure is secreted, which may be 

 important as a reef-former (Millepora). The stromatoporoids of 

 the Palaeozoic are believed to belong to this group. They repre- 

 sent enormous accumulations of lime taken by minute organisms 

 from the sea-water and built into their structures. These are often 

 heads of great size, some attaining a diameter of ten feet. 



The coral or anthozoan polyp is more complicated, there being, 

 in addition to the parts found in the hydroid polyp, an enteric sac. 

 or stoinodccuru, formed by invagination of the mouth area, and a 

 series of fleshy septa or mesenteries, dividing the body radially. 

 Many anthozoan polyps secrete a calcareous structure (coral) 

 which typically is characterized by a series of radially placed calcar- 

 eous plates or septa, variously united by transverse structures and 

 surrounded by one or more calcareous walls. In Palaeozoic time 

 these were built mostly on the plan of four and grew into isolated 

 horn-shaped structures on the broad septate end of which the 

 polyp rested (Tetrasepfata) . In later times to the present the plan 

 of six (Hexaseptata) or eight (Oetoseptata) became the dominant 

 one. and the forms became compound, so that in some modern 

 coral heads thousands of individual polyps participate. A fourth 

 group in which the septa were absent or represented by spines oidy. 

 while the walls were provided with pores (Aseptata), was chiefly 

 confined to the Palaeozoic. The reproduction of the Anthozoa is 

 carried on by fission and by ova. 



Phylum IV — jMolluscoidea. The Molluscoidea comprise two 

 classes which are widely different in their external adult charac- 

 ters but closely similar in their early life history. The Br\ozoa 

 are commonly compound aquatic forms, either encrusting other ob- 

 jects or forming solid masses not unlike in form to some early 

 corals, with which they have sometimes been united. The colony, or 

 soarium, consists of cells (aooccia) generally of lime and loosely or 

 closely aggregated, in the latter case often becoming prismatic. 

 They are hollow or divided by transverse calcareous partitions or 

 dissepiments and have various other structures. Smaller tubes 

 (mesopores) are present in some cases. Colonially the Bryozoa 

 may constitute a solid mass or head, a flat expansion, a network, 

 in which large open spaces are left between series of zooecia (as in 

 Fenestella, etc.), or a great variety of other forms. In Palaeozoic 



