384 SYDNEY J. HTOKSON. 



vesicle, t.c. Interstitial cells of the ectoderm. ;«. Muscles. ?//i?.s. Mesogloea. 

 mf. Mesenterial filameuts. nem. Neniatocyst. n.p. Nerve plexus. P.Msc. 

 Protractor muscles. R. Msc. Retractor muscles. Si. Siphonoglyplie. sp. 

 Spicules. 5jo. c. Spicule forming cells. aS"^. Stomodajum. */?.y. Spermagem. 

 t. Tentacles. V. Ventral iutermeseuterial space, v. m.f. Ventral mesenterial 

 filaments, y. s. Yolk spaces in the ovum. 



PLATE 36. 



Fig. 1. — A specimen of Alcj'ouium digitaiuin in the Cambridge 

 Museum. 



Fig. 2. — A specimen of Alcyouium glomeratum from the Marine Bio- 

 logical Association's laboratory at Plymouth. Both of these illustrations were 

 drawn from spirit specimens, and are for the purpose of illustrating the dif- 

 ferences of external form of the two species as may be seen in ordinary 

 museum specimens. 



Fig. 3. — Spicules of Alcyonium digitatum (white or pink varieties) 

 X 240. 



Fig. 4. — Spicules of A. digitatum (yellow variety) x 240. 



Fig. 5. — Spicules of A. glomeratum x 240. 



PLATE 37. 



Fig. 6. — A fully expanded polyp drawn from a living specimen. The bases 

 of the tentacles are extended in a bullate manner, and the pinnae are partly 

 withdrawn. The stomodseum {St.) and the six short and two long mesenterial 

 filaments may be seen through tlie transparent body-wall. At the base of the 

 crown of tentacles, and at the base of the extensible portion of tiie polyp, tlie 

 body-wall is more opaque, and contains clusters of spicules. 



Fig. 7. — Vertical section through a portion of a colony (semi-diagrammatic) 

 to show the polyps in different stages of retraction, the different sizes of the 

 polyps, the canal system, the distribution of the spicules, &c. The mesen- 

 terial filaments are represented as being in the same plane as the tentacles, 

 which they are not, for the sake of illustrating certain points in the arrange- 

 ment of the structure. The following points are illustrated in this figure : — 

 1' represents a fully expanded polyp. 2' represents a partially retracted polyp 

 in which the tentacles are contracted and folded over towards the centre of 

 the disc; the body-wall forms a circular fold over the crown of tentacles. 3' 

 represents a polyp in which the tentacles and stomodseum have sunk to the 

 level of the general surface of the colony. 4' represents a polyp in which these 

 organs have sunk below the level of the general surface of the colony. 5' 

 represents a polyp which is completely retracted, tlie thin body-wall of the 

 extensible portion having sunk below the surface, and the densely spiculated 



