286 
W. M. Wheeler 
dorsal cirri of Chaetopods, is highly probable. If, then, we take 
into consideration tbe segmental sacs and their homology with tbe 
dorsal cirri of neural parapodia — a theoiy advanced by Eisig ('87) 
and siipported by bis own observations and those of Ehlers ('64) and 
Claparede ('68) on species of Glycera^ in wbicli it is possible to 
trace tbe gradual reduction and transformation of tlie 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 (usnally absent) 
neural parapodiiun .... parapodium 
lateral line organ .... segmental sac 
(modified dorsal cirrus of neural parapodium) 
baemal parapodium .... absent 
dorsal cirrus two or more dorsal cirri. 
Tbe second point to wbich I would call attention, concerns tbe 
secretion of tbe books, reserve hooks and supporting rods of tbe 
parapodia in Myzostoma as compared witb tbe secretion of tbe setae 
in Chaetopods. The minute structure of tbe books etc. in Myzostoma 
is tbus described by Nansen ('85, pag. 77): »The hooks are not, as 
Graff States in bis monograpb, bollow, but consist of two layers; 
an outer, somewbat homogeneous 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 tbe base of tbe hooks, but 
increases, gradually, in tbickness towards tbe extremity, and tbe 
acuminate extremities of tbe hooks become formed, tbus, almost 
exclusively by this layer, and acquire, also, a more intense yellow 
colour, especially in tbe more developed hooks. Tbe inner fibrous 
mass consists of colourless fibres, wbicb are thickest in tbe center 
of tbe hook and, on transverse sections, exhibit a distinct hexagonal 
form, vide PI. 7 Fig. 19. Towards tbe outer layer tbese hexagonal 
fibres become so minute tbat tbey are with difficulty distinguisbable. 
Tbis fibrous mass is, usually, vividly stained by colouring reagents. 
Tbe structure of tbe supporting rod is similar to tbat of tbe hooks, 
and tbe manubrial plate is composed of tbe homogeneous yellow 
substance of tbe outer layer.« 
This description, wbicb I am able to confirm from my own 
Observation, sbows clearly tbat there is no essential difference between 
