286' 
W. M. Wheeler 
dorsal cirri of Chaetopods, is highly probable. If, then, we take 
into consideration tbe segmental sacs and tbeir homology witb tbe 
dorsal cirri of neural parapodia — a theory advanced by Eisig ('87) 
and supported by bis own observations and tbose of Ehlers ('64) and 
Claparède ('68) on Speeles of Glycera^ in wbich it is possible to 
trace the graduai reduction and transformation of tbe cirri into 
organs like those of tbe lateral line — we bave tbe following homo- 
logies between tbe Cbaetopoda and Myzostomidae : 
Cbaetopoda Myzostoma 
ventral cirrus ventral cirrus (usually absent)^ 
neural parapodium .... parapodium 
lateral line organ .... segmentai sac 
(modified dorsal cirrus of neural parapodium) 
baemal parapodium .... absent 
dorsal cirrus two or more dorsal cirri. 
Tbe second point to wbicb I would cali attention, concerns tbe 
secretion of tbe books, reserve books and supporting rods of tbe 
parapodia in Myzostoma as compared vs^itb tbe secretion of tbe setae 
in Cbaetopods. Tbe minute structure of tbe books etc. in Myzostoma 
is tbus described by Nansen ('85, pag. 77): »Tbe books are not, as 
Graff States in bis monograpb, bollow, but consist of two layers; 
an outer, somewbat bomogeneous layer and an inner one composed 
of fibrous substance. Tbe outer layer bas a yellow colour and is 
very sligbtly staining; it is very tbin at the base of tbe books, but 
increases, gradually, in thickness towards the extremity, and the 
acuminate extremities of the books become formed, tbus, almost 
exclusively by this layer, and acquire, also, a more intense yellow 
colour, especially in tbe more developed books. The inner fibrous 
mass consists of colourless fìbres, wbicb are thickest in the center 
of the hook and, on transverse sections, exbibit a distinct hexagonal 
form, vide PI. 7 Fig. 19. Towards tbe outer layer these hexagonal 
fibres become so minute that they are with difficulty distinguishable. 
This fibrous mass is, usually, vividly stained by colouring reagents. 
The structure of the supporting rod is similar to that of the books, 
and the manubrial piate is composed of tbe bomogeneous yellow; 
substance of the outer layer.« 
This description, wbich I am able to confirm from my own 
Observation, shows clearly that there is no essential ditference between 
