pseudoacephalus

pseu·do·a·ceph·a·lus

(sū'dō-ă-sef'ă-lŭs),
An apparently headless placental parasitic twin that, however, has rudimentary cephalic structures that can be demonstrated by dissection.
[pseudo- + G. a- priv. + kephalē, head]

pseudoacephalus

(soo″dō-ā-sĕf′ă-lŭs) [″ + a-, not, + kephale, head]
A parasitic twin that has a rudimentary cranium.