HISTOLOGY OF ASTRAEIDS 111 



soft parts of the A s t r a e i d a e would also explain the absence 

 of a nervous system — central or peripheral. 



In the larval stage the middle lamina is present everywhere 

 and is fibrous, though thinner than in the polyp. Pleats are 

 hardly recognizable in mesenteries, but specialized fibres are 

 present. H. Y. Wilson found that in the development of 

 Manicina areolata the middle lamina appears in the 

 solid planula stage (37, fig. 5). This observation is corroborated 

 by my study of the solid embryonic stages occurring in the 

 coelenteric cavities of polyps of Fa via fragum. 



The middle lamina of Actinians as seen from a study of serial 

 sections of young polyps of Sagartia bellis, Metridium 

 senilis, and Corynactis viridis (which in alcohol 

 measured2mm. x 1 mm. ,12mm. x 3mm., and 4mm. x 3-5 mm.), 

 obtained from Plymouth, is in essential points similar to that 

 of Astraeid corals. In the former two species, the middle 

 lamina has a swollen somew^hat loosely spongy core containing 

 many nuclei which is bounded by closely arranged unbranched 

 fibres ; the plexus of the spongy core consists of branching 

 fibres which are more abundant than in coral polyps (fig. 16). In 

 Corynactis viridis the meshwork is closer, approaching 

 the condition in the Astraeidae. The principal difference 

 is that, in the column-w^all of Actinians, the middle lamina is 

 considerably thickened. The fibrous condition of the middle 

 lamina of various Z o a n t h a r i a had been previously observed 

 by Kolliker, Schneider, and Eotteken, von Heider, Jourdan, 

 the Hertwigs, and Faurot.^ As early as 1875, Allman remarked 

 that the ' hyaline lamella ' (= middle lamina) of Myriothela 

 consisted of ' two layers — internally a perfectly transparent, 

 thin, structureless membrane, and externally a layer of fibrillae, 

 which adheres closely to the structureless membrane ' (2, 

 p. 554, fig. 6). 



The histology of the middle lamina of the Madreporaria 

 resembles that of mammalian connective tissue, the massed 



^ Hickson in 1883 described the middle lamina of Tubipora as 

 consisting of a ' homogeneous matrix ' in which might be found ' cells 

 and fibres ' (17, p. 11). 



