EEPOET OjST THE MYZOSTOMIDA. 
25 
Caudal appendages round, witH terminal threads, 
* * Mouth (and cloaca) dorsal. 
With large, prominent, ventral elevation, -without suckers, 
Body thin, without central elevation, with stalked suckers, 
* Living in cysts or forming malformations of the pinnules, without suckers. 
Living on the pinnule which becomes enlarged on the ventral side, . 
Living in the interior of cysts. 
Androgynous, all individuals equally formed. 
Border with cirri, cysts as thickenings of the arms, 
Border without cirri, cysts as swollen pinnules, 
Sexes separated, large female and dwarf male individuals. 
Cysts as intumescences of the arm-joints (border of the body with 
cirri), ........ 
Cysts as spirally rolled-up pinnules (border of the body with cirri), . 
Cysts as independent new formations of the skin of the host (border 
of the body without cirri). 
Female often thicker in the middle than in the lateral parts, cysts 
not stalked, . . . . ... 
Female of uniform thickness, cysts mostly stalked. 
Male with ramified testis, ..... 
Male with compact testis, ..... 
II. Intestine straight, not ramified (without suckers). Family and genus Stele- 
chorus, only one species, . ...... 
M. intermedium. 
M. pulvinar. 
M. calycotyle. 
M. asymmetricum. 
M. pentacrini. 
M. deformator. 
M. tenuispinum. 
M. willemoesii. 
M. eysticolum. 
M. inflator. 
M. murrayi. 
St. hyocrini. 
To this table I may be permitted to add some remarks about the natural affinities of the 
different species. The starting-point of the whole group is doubtless the Tardigrade-like form 
Stelechopus hyocrini. From some form like this Myzostoma folium has been developed ; 
it is characterised by an extended worm-like body, the absence of suckers, and by 
an arrangement of the parapodia, similar to that of Stelechopus. The ancestral form, 
however, had in all probability muscular septa placed at right angles to the long axis of the 
body and not radially. Out of these other elongated forms with suckers were developed, 
which in their turn gave rise to the more discoid species. 
A greater or less degree of mobile power may have produced the two different groups 
already alluded to, one of which is distinguished by its thin body and hyaline marginal 
portion, by the feeble development of the parapodia and their enclosed structures, and by 
the length of the cirri ; the other by the massive form of its body, and by the great 
development of parapodia and muscles, and the short cirri, which are sometimes altogether 
absent. 
In some species of the second group the change in position of the cloaca (. Myzostoma 
c datum and Myzostoma glabrum), or of mouth and cloaca [Myzostoma pulvinar and 
Myzostoma calycotyle ) to the dorsal surface, is to be looked upon as having been produced 
by the firm attachment of the parasite to the skin of its host by the great development 
of parapodia and suckers. 
In the first group we find differences in the number of the cirri, some having twenty, 
(ZOOL. CHALL. EXP. PART XXVII. 1884.) Dd 4 
