914 The American Naturalist. [October, 
group, — that is that they have had a common origin and descent (are ho- 
mophyletic). Other structures which appear very similar, and have often 
been regarded as homologous, are but analogous, and are to be regarded 
as homoplastic. Besides these there are others which are intermediate in 
position, being partly homophyletic and partly homoplastic. Among 
these are to be enumerated the musculature, a part of the nervous and 
water-vascular systems of the Holothurians. Semon also refers to the 
unanimity of results arrived at by him in his previous work on the 
Synaptidfe {Jenaische Zeitschrift, 1888), and those of the brothers 
Sara-sin in their studies of the Echinotheridae, both concluding that 
the Holothurians represent the stem form within the group of Echino- 
derms. Semon does not agree with Newmayr in assigning this position 
to the Cystidians. 
The Ontogeny of Pelvic and Shoulder Girdles.— R. Wieders- 
heim has studied the development of the girdles in Scyllium cani- 
ciila, Mustelus Icevis, Pristiurus nielanostomus , Torpedo ocellata, Thy- 
mallits vulgaris, Triton helveticus, T. alpestris, Siredofipisciformis, Sala- 
mandra maculata, Alytes obstetricans, Rana temporaria, R. esculenta, 
Lacerta agilis, Chelone midas, and Crocodilus biporcatus. His genera 
conclusions (^Ant. Anz. IV.) are as follows : ^ 
1. The pelvic and shoulder girdles are strictly homologous ; both 
possess the same Anlage. 
2. Both are phylogenetically and ontogenetically later formations 
than the free limbs. 
3. The free limb is to be regarded as the mechanical principle under 
the formative influence of which a yoking apparatus or fixation point 
must arise in the body wall. These structures are the s .oulder and 
pelvic girdles. 
4. In the Selachii, and apparently in all fishes, the primitive con- 
tinuity of the girdle and appendage persists into the cartilaginous con- 
in the precartilage condition. The cartilaginous Anlage are se{)arate 
in each rcu;i()n. though there may be in the Amphibia a secondary 
(transitory or permanent) fusion of the free-extremities with the 
girdlo. 
5. Prom the Amphibia on there is a marked tendency for the sepa- 
rate elements of the girdles to appear separate in the cartilaginous 
stage. The fusion is secondary, and earlier authors have been in 
error when describing a single hyaline cartilage blastoma. 
