EEPOET ON THE CRINOIDEA. lOl 



in vertical sections of a disk that is moderately free from pigment. In none of the 

 species of Pentctcrinus or Comatida which I have examined, have I failed to observe 

 this connection satisfactorily ; and in some fortunate sections the vascular structure may 

 be traced right down from the oral ring into the lower and more compact portion of the 

 plexiform gland (PL LXIL). This is generally more or less lobulated in form, and in 

 this respect very striking differences exist among the various tjrpes of Crinoids. 



Both in Rhizocrinus (PI. Villa, figs. 7, 8, x), and in Bathycrinus (PI. Vllb. 

 figs. 1, 4-8, x) it is almost as simple as in the Pentacrinoid. It is oval-oblong in section 

 and has but slightly developed irregularities of outline. In Antedon rosacea, and in the 

 lower part of the disk of Pentacrinus, untd it commences to subdivide, it is more irrefnilar 

 in form (PI. LVIII. fig. 4, x) ; and it shows indications of its lobular structui-e, as is well 

 seen in Lud'^'ig's admirable illustrations ' of the former type. I have made no horizontal 

 sections of the disk of Antedon eschrichti, but imagine that its plexiform gland must be 

 not very unlike that of PromachocHnus kerguelensis. Figs. 5 and 6 on PI. LVIII. represent 

 horizontal sections through the gland in the lower half of the disk of this type, fig. 6 

 being that nearer the calyx. The u'regularity in the form of the organ is very striking. 

 The upper part of the same disk was cut into vertical sections. They show the extreme 

 subdivision of the ventral end of the plexiform gland, and the termination of its branches 

 in the well defined labial plexus, the spongy part of which is represented in PL LIX. 

 fig. 9, XV. 



The minute structure of the plexiform gland of the Crinoids is as yet unknown. 

 According to Perrier ■ it is identical with that of the same organ in other Echiuotlerms. 

 Fresh specimens, and not spirit-material, are absolutely essential for its elucidation ; but 

 as Perrier and Koehler, who have both studied the plexiform gland of the Urchius, give 

 accounts of its structure which difier from one another, and from Apostolid^s' description 

 of the same organ in Ophiurids, there is a difificulty in determining from analog}^ and 

 stdl more so from observation, what its real nature is in Crinoids.^ Certain peculiarities 

 that I have noticed in the appearance of its lower portion in Pentacrinus decorus lead 

 me to think that Koehler's account of it in the Urchins * is probably more correct than 

 those of his fellow-workers. 



Upon one point, however, I am quite satisfied. Although the condition of my material 

 entirely precluded any minute observations on the structure of the plexiform gland, I 

 have had no difficulty in tracing its connection with the intervisceral blood-vessels of the 

 lower part of the disk (PI. LVII. fig. 2, PI. LVIII. fig. 6). This point was first noticed by 

 Ludwig in Antedon rosacea, and he illustrated it with an excellent figure.^ I have 



' Crinoideeu, loc. ciL, Taf. xviii. figs. 57-59. 



2 Sur rorganisation des Crinoides, Compfes rendm, t. xcvii., No. 3, p. 18. 



' For a discussion of this subject see Quaii. Jouni. Micr. ScL, N. S., vols, xxii.-.xjciv., 1882-1884. 



* Recherches sur les i;chimdes des Cotes de Provence. Ann. du Mus. d'Hist. Nat. de Marseille, t. i. pp. 73-77, 95-99. 



^ Crinoideen, loc. cit., Taf. xviii. fig. 59. 



