184 PHYLUM ECHINODERMATA. 



The small sac beneath the madreporite (Fig. 132, mv), sometimes called 

 the ampulla, is the small right hydrocoel (p. 144). 



The discovery of the communication between the upper end of the 

 stone-canal and the axial sinus, we owe to the work of Perrier, Durham, 

 and MacBride. The number of madreporites and stone-canals varies 

 considerably. In the majority of cases there is only one of each, but there 

 may be more, and this increase in their number is usually, though not 

 always, associated either with an increase in the number of arms, whether 

 such appears as an individual variation or as a constant specific character, 

 or with the power of asexual reproduction by fission across the disc 

 which some starfishes possess. Though the increase in the number of 

 madreporites is generally associated with a greater number of arms than 

 five, it is by no means always found in such forms. For instance, in the 

 genera Heliaster and Labidiaster which normally have a large number of 

 arms there is only a single madreporite. 



The increase in the number of madreporites is found most frequently 

 in the families Asteriidae, Stichasteridae, Echinasteridae, and Linckiidae, 

 and the following table* in which the abbreviation M is used for madre- 

 porite shows some of the most conspicuous instances of it : 



Asterias calamaria Gray, with 6-12, usually 7 arms, has in 7-armed 

 forms usually 1, rarely 2 M ; but in a 12-armed specimen 2 M and in 

 an 11-armed one, 4 M have been observed. 



Asterias tenuispina Lam., has in 5- to 9-armed forms 1 to 3, rarely 4 M. 



Asterias capensis Perr., has 6 (rarely 5) arms and 3 M. 



Asterias rubens L., exceptionally 2 M in 5-armed forms. 



Stichaster polyplax M.Tr., has 7 arms and 1 to 5, usually 3 M. 



Stichaster albulus Stimps., has 5 to 7 arms and 1 to 2 M. 



Acanthaster echinites Ellis and Solander, has in 13- to 20-armed speci- 

 mens 5 to 16 M. 



Acanthaster ellisii Gray, has with 11 to 19 arms 5 to 15 M. 



Echinaster eridanella M.Tr. and E. purpureus Gray, have in 5-armed 

 individuals 1 M, in 6- or 7-armed 2 M. 



Ophidiaster germani Perr., has in 5-armed forms 2 M. 



Linckia multifora Lam., has often 2 M in 5-armed specimens. 



Linckia pacifica Gray, var. diplax M. Tr., and L. guildingii Gray, have 

 as a rule 2 M in 4- to 7-armed specimens. 



In the above instances the madreporites are in different, either conti- 

 guous or remote, interradii, but cases are known in which there are two 

 madreporites and canals in the same interradius as an individual varia- 

 tion (Linckia multifora Lam., Heliaster multiradiata Gray, and in a 

 6-armed example of Asterias glacialis O.F.M.), and Giard has described 

 a specimen of Asterias rubens with one madreporite and two stone-canals 

 passing off from it. Finally it must be mentioned that sometimes the 

 madreporite is divided into several pieces, all however connected with 

 the same stone-canal. 



Tiedemann's bodies are small yellowish glandular bodies 

 attached to the inner wall of the circumoral vessel into which 

 they open (Figs. 135, 136). They consist of branching tubes of 



* Taken from Ludwig's excellent account in Bronn's Thierreich to 

 which the reader is referred for the facts and literature relating to the 

 variation in the number of madreporites. 



